I had this problem the other day and found the answer in a thread after no one helped.
Find the virtuemart.cfg.php file in adminstrator/components/com_virtuemart/
open it to edit and scroll down till you find the text below.
// these path and url definitions here are based on the Joomla! Configuration
below that text you will find 2 lines similar to the below...
define( 'URL', 'your site/' );
define( 'SECUREURL', 'your site/' );
edit the your site part with your website address.
That should solve it.