Add New Fields in joomla

alter table and add the new fields u want to the database.

Update set html elements so one can insert values for new fields in database from admin panel.
File: /administrator/components/com_virtuemart/html/product.product_form.php

Modify two functions: add() and update() and include your new fields in it so it can update and add new product to database in following file.
File: /administrator/components/com_virtuemart/classes/ps_product.php

This is how u can add new fields to your jos_vm_product table.