Hi,
Unfortunately it doesn't possible to show "out of stock" message right away (without clicking the add to cart button) in the category browse page. But it is possible in the "product details page" though.
In original Virtuemart, "add to cart" button is not visible in "category browse page" but we have enabled this by our VM2 Template, this is why the AJAX update for "stockable variants" when the variant is out of stock doesn't work succesfully in category browse page. But still, the functionality is there which avoid any confusions. Buyers can't add any variant to cart if it is out of stock. I know, it would better to show the "out of stock" text right away when it is selected, but we tried it, it didn't work out. Some core files of Virtuemart needed to be patched which is not recommended, because modifying the Virtuemart Core files is always a bad thing.
Regards.