I just spent way too much time looking for this answer. I found it here.

http://www.poweredbyvirtuemart.com/VirtueMart-Help/TOP-10-Solutions-to-common-VirtueMart-problems.html

Here is what the answer was:

Go to VirtueMart > Configuration > Site TAB > Layout > Configuration > Use Ajax to add, update or delete products from the cart? > No