Forget all that -- was thinking Darkmart
just installed and tried the plugin
you can see it here
test.flexiblewebdesign.com/virtuemart-2-template-puremart/index.php?option=com_virtuemart&view=category&virtuemart_category_id=1&Itemid=474
Citizen Men's Test2
I don't think you'll be impressed - it will need some work on it as in different views you get just the add to - in others the the dropdown etc
no checking that a choice has been made + the price is wrong (uses only the attribute price)
BUT
Our template is but a template - it just displays what VM sends it .. so you may get it to work - but obviously it isn't in our remit to get 3rd party plugins to work
If you look at any forum posts to do with add to cart on cat pages you will see that certainly out of the box - it doesn't work as soon as you have Cart attributes
Cheers