Hello,
Im having the same problem as described. I used Firebug to detect a conflict between declaration of jquery-min.js in template index.php and virtuemart/config.php. I used a factory template (bez50) and virtuemart was adding products to the cart normally. I decided to add some jquery functionality to the homepage which is not related with virtuemart so it was necessary the code <script>bla bla bla </script> to the template/index.php. After that the new functionality was working but the virtuemart "add to cart" button neither the plus and minus buttons.
I saw that this new wanted functionality was working when ever a virtuemart module was called without including the <script>bla bla bla </script> in the template/index.php
PLEASE HELP