Check to see if you have your template selected in the virtuemart template configuration page. if not, select it and that should solve your problem...
Just to help clarify the matter, here is the page that is correctly styled with my joomla template:
http://mandalaplants.com.au/index.php/component/virtuemart/clivias/clivia-detail?Itemid=470
And here is the page once you click 'add to cart':
http://mandalaplants.com.au/index.php/component/virtuemart/clivias/index.php
As you can see the template is suddenly unable to be found despite having ben used on the previous page.
Any thoughts?