Just so I understand what you are wanting to accomplish . . . . which sections of your third-party VirtueMart theme do you want to use other than the default theme sections/elements that were styled for Zeolite II? In other words, do you ONLY want to use the Zeolite II styling for the Cart Module, but the theme styling of your third-party theme for all other sections/elements?
I do know that you can specify which theme you want to use for your Category pages as well as the individual product pages (also called the “flypage”). I’m not 100% sure about selecting different themes for ALL elements of VirtueMart …. but then again, I’m also a proponent of the belief that there is virtually nothing one cannot accomplish with an open-source platform (such as Joomla and VirtueMart).
Incidentally, which other VirtueMart theme are you using? (there’s no shame in going outside of JA for certain elements you want to customize above/beyond the base JA template theme – especially for elements such as your ecommerce elements).