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...
I'm having a similar problem. I have selected 'MandalaPlants' (my main joomla template) as my 'default template for your shop' and saved the setting. The main page of my shop appears fine, but when I click on Add to cart and it moves to the next page ("Shopping Cart") it has the error "The choosen template couldnt found on the filesystem: MandalaPlants"
Is there some link to my templates that I am missing, or do I need to do some tricky code fixing somewhere?
Any thought that could help me and the OP would be appreciated,