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.
So you think, we just can replace the fancybox by the jQuery browser Plugin?
I didn't learn javascript yet so i can't answer you. This was a solution given from a friend programmer to solve a problem with the website https://e-aggelia.gr/αγγελίες/μικρές-αγγελίες/απλές-αγγελίες-
Before the modification it was not working. The second custom field that is depending on the first was not updated without refreshing the page.