i had problems with mutivariants products when i started the e-shop with vm 3.0.12. Some programmers told me there is compatibility problem with fancybox and jquery. Fancybox uses the variable "browser" that is depreciated from jquery 1.11 that i use. I had the same issue again when updated to vm 3.0.18. So if you have same problems you can try with the file i attach. Copy and replace components/com_virtuemart/assets/js/fancybox/jquery.fancybox-1.3.4.pack.js with the attached file after extracting it from the zip.