First of all. Please update template to the latest version. If this will not help please rename /html/com_virtuemart/productdetails/defult.php to /html/com_virtuemart/productdetails/_defult.php so make a test where the problem is.
Just to add one more note, I'm also using the latest version of Virtuemart. And to reiterate, if I select a different template for virtuemart, zoom and choosing variants works fine.
I'm afraid just a look at Your website was not enough. There are no error massags in console or anything which can lead to the cause of this problem. Please send me an access to FTP. Hopefully making a dump of variables will help to resolve it.
I could not found the cause for this issue, so I disabled our template just to check and the same problem exists even if no other extensions are used. This means it's Virtuemart related. You need to talk to Virtuemart developers or check their documentation. It may be just a misconfiguration, but again VM support will be more helpful here than I can be.
??? Of course it says "Choose Variant", but if you choose Colour Black and Size XL, you DO get "Add to Cart". Did you not actually choose your colour/size to test? I just disabled the template myself and this is what I get. http://www.awesomescreenshot.com/image/ ... ab7e96fe46
This means I didn't understood what the problem is. Please send me an access to joomla panel so I'll be able to switch back to our template and play with javascript options in VM settings.
I'm afraid I cannot find the cause of this problem. I've send Your access to our programmers. Hopefully they will have more luck with it. I'll let You know as soon as they will answer me.
Programmers found out that disabling javascript compression helps with this problem. Looks like some of the scripts cannot be compressed together. You will have to look for a file to be excluded from compression or just leave it disabled as it is now.