OpenGlobal VirtueMart Bulk Discount
This VirtueMart 2.x/3.x custom field plugin allows you to create discounts based on the quantity of a product that has been ordered.
Simply add this custom field to the relevant products and specify how many products need to be ordered and what the discount is and the discount will be automatically calculated when enough items are ordered. This is a great way to implement "Buy 3, Get 1 Free" -type deals.
Please note that this plugin is only compatible with VirtueMart version 2.0.12 and upwards.