the file that handles handles the rendering of the product details page & form is:
administrator\components\com_virtuemart\html\shop.product_details.php
it does so like this:
$addtocart = "<div>
<form action=\"". $mm_action_url."index.php\" method=\"post\" name=\"addtocart\" id=\"addtocart\">"
.$ps_product_attribute->list_attribute($product_id)
// added for the advanced attribute modification
.$ps_product_attribute->list_advanced_attribute($product_id)
// end added for advanced attribute modification
.$ps_product_attribute->list_custom_attribute($product_id);
// end added for custom attribute modification
so you would need to do some customisation/tweaking in the those list_*_attribute() functions to make them render the tables/rows differently. you'll find them in
\administrator\components\com_virtuemart\classes\ps_product_attribute.php