Hi,
Since there is not enough space and it didn't look good to include "add to cart" button in the GRID layout. We have decided to have the "add to cart" appears only on the thumbnail when mouse-hovered. This our decision and we developed it that way.
If you want to customize it and put a static "add to cart" button below the thumbnails, you would some coding changes in the category browse page PHP file, which is located here:
ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/category/default.php
(default.php, flexv2.php or whatever layout you chose from Virtuemart Configuration for category browse page).
Since this is how our VM Temlate was designed and what you are asking is a "custom modification". I can't give you the detailed instruction here. Coding changes are not easy thing to do. You should purchase our "premium support" and let us take care of it for you. 2 hours would be enough for your case:
www.flexiblewebdesign.com/Premium-Support/Virtuemart-Premium-Support
On the other hand, you can use the LIST layout as default though. LIST layout (defaullist or flexv2list) has the "add to cart" button statically in every product (it is simply because there is much more space and we included it.)
Regards.
Forum Support.