For this Youtube plugin Virtuemart extension, all you need to to is to install the plugin normally like a Joomla plugin then simply copy and paste a Youtube link into a product description. It doesn't require you to be hacking. The Virtuemart extension is compatible with all Virtuemart versions and works in custom fields of Virtuemart version 2 xxx
How to install Virtuemart Youtube plugin for use?
Step 1: Download the plugin and install it like a Joomla installation
Step 2: Enable the plugin in Joomla administration
Step 3: Go to Virtuemart -> Products -> Custom Field -> Create a new field and give it a name as “Youtube” -> choose Youtube plugin in the section “Selet a plugin” -> Save.
From Step 1 to 3: Your installation and configuration are at once. You don’t need to perform them again.
Step 4: Go to a product -> Custom Field -> Here, you choose Youtube from the Custom Field list -> Copy Youtube URL -> Type the title -> Adjust the width and height for Youtube video if it’s necessary -> Save.
Youtube video will be displayed on the product description page.