You would need a code hack in this JS files;
/components/com_virtuemart/assets/js/vmprices.js
to disable the "Modal Popup". (but enable the "show modal popup" parameter in VM Configuration).
(comment out the lines to load the modal popup -- Facebox or Fancybox, which ever you are using currently)
Since it is a custom modification, it is beyond our free support. I can't guide you step-by step.
It is not recommended to touch vmsite.js though, because it is a very important file and takes care the core functions for adding product to cart. if you insist, then you can raise this question in
forum.virtuemart.net
Regards,
Support