Magento2 : How to change product thumbnail image gallery horizontal to vertical?

You can configure slider in view.xml file.
It located in /app/design/frontend/vendor_name/theme_folder/etc/view.xml.
There you can write vars for gallery. For luma theme it looks like this
change this tag <var name="navdir">horizontal</var> 


-------------------------------------------------------------------------------------------
<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
    <media>
        <images module="Magento_Catalog">
            <image id="bundled_product_customization_page" type="thumbnail">
                <width>140</width>
                <height>140</height>
            </image>
            <image id="cart_cross_sell_products" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="cart_page_product_thumbnail" type="small_image">
                <width>165</width>
                <height>165</height>
            </image>
            <image id="category_page_grid" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="category_page_grid-1" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="category_page_list" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="customer_account_my_tags_tag_view" type="small_image">
                <width>100</width>
                <height>100</height>
            </image>
            <image id="customer_account_product_review_page" type="image">
                <width>285</width>
                <height>285</height>
            </image>
            <image id="customer_shared_wishlist" type="small_image">
                <width>113</width>
                <height>113</height>
            </image>
            <image id="gift_messages_checkout_small_image" type="small_image">
                <width>75</width>
                <height>75</height>
            </image>
            <image id="gift_messages_checkout_thumbnail" type="thumbnail">
                <width>100</width>
                <height>100</height>
            </image>
            <image id="mini_cart_product_thumbnail" type="thumbnail">
                <width>75</width>
                <height>75</height>
            </image>
            <image id="new_products_content_widget_grid" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="new_products_content_widget_list" type="small_image">
                <width>270</width>
                <height>340</height>
            </image>
            <image id="new_products_images_only_widget" type="small_image">
                <width>78</width>
                <height>78</height>
            </image>
            <image id="product_base_image" type="image">
                <width>265</width>
                <height>265</height>
            </image>
            <image id="product_comparison_list" type="small_image">
                <width>140</width>
                <height>140</height>
            </image>
            <image id="product_page_image_large" type="image"/>
            <image id="product_page_image_medium" type="image">
                <width>700</width>
                <height>700</height>
            </image>
            <image id="product_page_image_small" type="thumbnail">
                <width>88</width>
                <height>110</height>
            </image>
            <image id="product_swatch_image_large" type="image"/>
            <image id="product_swatch_image_medium" type="image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="product_swatch_image_small" type="thumbnail">
                <width>88</width>
                <height>110</height>
            </image>
            <image id="product_page_main_image" type="image">
                <width>700</width>
                <height>560</height>
            </image>
            <image id="product_page_main_image_default" type="image">
                <width>700</width>
                <height>560</height>
            </image>
            <image id="product_page_more_views" type="thumbnail">
                <width>88</width>
                <height>110</height>
            </image>
            <image id="product_stock_alert_email_product_image" type="small_image">
                <width>76</width>
                <height>76</height>
            </image>
            <image id="product_small_image" type="small_image">
                <width>135</width>
                <height>135</height>
            </image>
            <image id="product_thumbnail_image" type="thumbnail">
                <width>75</width>
                <height>75</height>
            </image>
            <image id="recently_compared_products_grid_content_widget" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="recently_compared_products_images_names_widget" type="thumbnail">
                <width>75</width>
                <height>90</height>
            </image>
            <image id="recently_compared_products_images_only_widget" type="thumbnail">
                <width>76</width>
                <height>76</height>
            </image>
            <image id="recently_compared_products_list_content_widget" type="small_image">
                <width>270</width>
                <height>340</height>
            </image>
            <image id="recently_viewed_products_grid_content_widget" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
            <image id="recently_viewed_products_images_names_widget" type="small_image">
                <width>75</width>
                <height>90</height>
            </image>
            <image id="recently_viewed_products_images_only_widget" type="small_image">
                <width>76</width>
                <height>76</height>
            </image>
            <image id="recently_viewed_products_list_content_widget" type="small_image">
                <width>270</width>
                <height>340</height>
            </image>
            <image id="related_products_list" type="small_image">
                <width>152</width>
                <height>190</height>
            </image>
            <image id="review_page_product_image" type="small_image">
                <width>285</width>
                <height>285</height>
            </image>
            <image id="rss_thumbnail" type="thumbnail">
                <width>75</width>
                <height>75</height>
            </image>
            <image id="sendfriend_small_image" type="small_image">
                <width>75</width>
                <height>75</height>
            </image>
            <image id="shared_wishlist_email" type="small_image">
                <width>135</width>
                <height>135</height>
            </image>
            <image id="side_column_widget_product_thumbnail" type="thumbnail">
                <width>75</width>
                <height>90</height>
            </image>
            <image id="upsell_products_list" type="small_image">
                <width>152</width>
                <height>190</height>
            </image>
            <image id="wishlist_sidebar_block" type="thumbnail">
                <width>75</width>
                <height>90</height>
            </image>
            <image id="wishlist_small_image" type="small_image">
                <width>113</width>
                <height>113</height>
            </image>
            <image id="wishlist_thumbnail" type="small_image">
                <width>240</width>
                <height>300</height>
            </image>
        </images>
    </media>
    <vars module="Magento_Catalog">

        <!-- Gallery and magnifier theme settings. Start -->
        <var name="gallery">
            <var name="nav">thumbs</var> <!-- Gallery navigation style (false/thumbs/dots) -->
            <var name="loop">true</var> <!-- Gallery navigation loop (true/false) -->
            <var name="keyboard">true</var> <!-- Turn on/off keyboard arrows navigation (true/false) -->
            <var name="arrows">true</var> <!-- Turn on/off arrows on the sides preview (true/false) -->
            <var name="caption">false</var> <!-- Display alt text as image title (true/false) -->
            <var name="allowfullscreen">true</var> <!-- Turn on/off fullscreen (true/false) -->
            <var name="navdir">horizontal</var> <!-- Sliding direction of thumbnails (horizontal/vertical) -->
            <var name="navarrows">true</var> <!-- Turn on/off on the thumbs navigation sides arrows(true/false) -->
            <var name="navtype">slides</var> <!-- Sliding type of thumbnails (slides/thumbs) -->
            <var name="transition">
                <var name="effect">slide</var> <!-- Sets transition effect for slides changing (slide/crossfade/dissolve) -->
                <var name="duration">500</var> <!-- Sets transition duration in ms -->
            </var>
            <var name="fullscreen">
                <var name="nav">thumbs</var> <!-- Fullscreen navigation style (false/thumbs/dots) -->
                <var name="loop">true</var> <!-- Fullscreen navigation loop (true/false/null) -->
                <var name="arrows">true</var> <!-- Turn on/off arrows on the sides preview in fullscreen (true/false/null) -->
                <var name="caption">false</var> <!-- Display alt text as image title in fullscreen(true/false) -->
                <var name="navdir">horizontal</var> <!--Sliding direction of thumbnails in fullscreen(horizontal/vertical)  -->
                <var name="navarrows">false</var> <!-- Turn on/off on the thumbs navigation sides arrows(true/false) -->
                <var name="navtype">slides</var> <!-- Sliding type of thumbnails (slides/thumbs) -->
                <var name="transition">
                    <var name="effect">slide</var> <!-- Sets transition effect for slides changing (slide/crossfade/dissolve) -->
                    <var name="duration">500</var> <!-- Sets transition duration in ms -->
                </var>
            </var>
        </var>

        <var name="magnifier">
            <var name="fullscreenzoom">20</var>  <!-- Zoom for fullscreen (integer)-->
            <var name="top"></var> <!-- Top position of magnifier -->
            <var name="left"></var> <!-- Left position of magnifier -->
            <var name="width"></var> <!-- Width of magnifier block -->
            <var name="height"></var> <!-- Height of magnifier block -->
            <var name="eventType">hover</var> <!-- Action that atcivates zoom (hover/click) -->
            <var name="enabled">false</var> <!-- Turn on/off magnifier (true/false) -->
            <var name="mode">outside</var> <!-- Zoom type (outside/inside) -->
        </var>

        <var name="breakpoints">
            <var name="mobile">
                <var name="conditions">
                    <var name="max-width">767px</var>
                </var>
                <var name="options">
                    <var name="options">
                        <var name="nav">dots</var>
                    </var>
                </var>
            </var>
        </var>
        <!-- end. Gallery and magnifier theme settings -->

        <var name="product_small_image_sidebar_size">100</var>  <!-- Override for small product image -->
        <var name="product_base_image_size">275</var>           <!-- Override for base product image -->
        <var name="product_base_image_icon_size">48</var>       <!-- Base product image icon size -->

        <var name="product_list_image_size">166</var>           <!-- New Product image size used in product list -->
        <var name="product_zoom_image_size">370</var>           <!-- New Product image size used for zooming -->

        <var name="product_image_white_borders">1</var>
    </vars>
    <vars module="Magento_Bundle">
        <var name="product_summary_image_size">58</var>         <!-- New Product image size used for summary block-->
    </vars>

    <vars module="Magento_ConfigurableProduct">
        <var name="gallery_switch_strategy">prepend</var>
    </vars>

    <vars module="Js_Bundle">
        <var name="bundle_size">1MB</var>
    </vars>
    <exclude>
        <item type="file">Lib::jquery/jquery.min.js</item>
        <item type="file">Lib::jquery/jquery-ui-1.9.2.js</item>
        <item type="file">Lib::jquery/jquery.details.js</item>
        <item type="file">Lib::jquery/jquery.hoverIntent.js</item>
        <item type="file">Lib::jquery/colorpicker/js/colorpicker.js</item>
        <item type="file">Lib::requirejs/require.js</item>
        <item type="file">Lib::requirejs/text.js</item>
        <item type="file">Lib::legacy-build.min.js</item>
        <item type="file">Lib::mage/captcha.js</item>
        <item type="file">Lib::mage/dropdown_old.js</item>
        <item type="file">Lib::mage/list.js</item>
        <item type="file">Lib::mage/loader_old.js</item>
        <item type="file">Lib::mage/webapi.js</item>
        <item type="file">Lib::mage/zoom.js</item>
        <item type="file">Lib::mage/translate-inline-vde.js</item>
        <item type="file">Lib::mage/requirejs/mixins.js</item>
        <item type="file">Lib::mage/requirejs/static.js</item>
        <item type="file">Magento_Customer::js/zxcvbn.js</item>
        <item type="file">Magento_Catalog::js/zoom.js</item>
        <item type="file">Magento_Ui::js/lib/step-wizard.js</item>
        <item type="file">Magento_Ui::js/form/element/ui-select.js</item>
        <item type="file">Magento_Ui::js/form/element/file-uploader.js</item>
        <item type="file">Magento_Ui::js/form/components/insert.js</item>
        <item type="file">Magento_Ui::js/form/components/insert-listing.js</item>
        <item type="directory">Magento_Ui::js/timeline</item>
        <item type="directory">Magento_Ui::js/grid</item>
        <item type="directory">Magento_Ui::js/dynamic-rows</item>
        <item type="directory">Magento_Ui::templates/timeline</item>
        <item type="directory">Magento_Ui::templates/grid</item>
        <item type="directory">Magento_Ui::templates/dynamic-rows</item>
        <item type="directory">Magento_Swagger::swagger-ui</item>
        <item type="directory">Magento_Tinymce3::tiny_mce</item>
        <item type="directory">Lib::modernizr</item>
        <item type="directory">Lib::tiny_mce</item>
        <item type="directory">Lib::varien</item>
        <item type="directory">Lib::jquery/editableMultiselect</item>
        <item type="directory">Lib::jquery/jstree</item>
        <item type="directory">Lib::jquery/fileUploader</item>
        <item type="directory">Lib::css</item>
        <item type="directory">Lib::lib</item>
        <item type="directory">Lib::extjs</item>
        <item type="directory">Lib::prototype</item>
        <item type="directory">Lib::scriptaculous</item>
        <item type="directory">Lib::less</item>
        <item type="directory">Lib::mage/adminhtml</item>
        <item type="directory">Lib::mage/backend</item>
    </exclude>
</view>