You need to import the cart functions into the category browsepage. CAE need them, the entire hack relies on cart related functionality. by default in the category browsepage the needed functions aren't used. A bit tricky, but can be done.
First of all, you need to change some settings, like Virtuemart --> Configuration --> Site > Layout > Configuration > Show the Add-to-cart-button on the product list is set to YES, - this enables the use of a simple add-to-cart button on category browsepage, but this is crucial, then you need to import couple of needed libraries. I never did this to the end myself, but I saw some posts around here in the forum.