Hi,
This is considered as 100% custom modifications
(because you want to change how the things work/look) and unfortunately we don't provide free support or step-by-step instructions for custom coding modifications.
You may purchase premium support:
www.flexiblewebdesign.com/premium-support
and let us take care this modification for you directly on your website though.
TIP: You need to make changes into these files:
Category:
/templates/YOUR_TEMPLATE/html/com_virtuemart/category/default.php
Product Details:
/templates/YOUR_TEMPLATE/html/com_virtuemart/productdetails/default.php
Regards.
Support