If you read the documentation on creating a custom product details template for Virtuemart 3.x (and Joomla 3.x of course), you might find it sounding complicated. But it is not at all... at least not if you're used to creating template overrides for other modules and components.
- First, copy the default template that's probably currently being used at [front end] \components\com_virtuemart\views\productdetails\tmpl\default.php. Open default.php and copy the contents into another folder (call it what you want). I called mine tjohns.php.
- Then, in your template folder, create the file path if it's not created already for other overrides you may have made: [front end] \templates\[your template]\html\com_virtuemart\productdetails\tjohns.php (instead of tjohns.php, it would be whatever you called the file).
- In the Virtuemart administration, go to 'configuration' > 'templates' tab. You'll see that the field, 'product layout' is set to 'default'. You can now select the template that you put in your templates folder. In my case, I now have an option for 'tjohns'.
- Take the new template file you added to your template - and edit away!
I haven't tried so I can't say this will work 100%... but you should also be able to do the same thing with the category layout and cart pages.