Most likely the 'official' answer as of right now would be:
No, vBGallery is not supported on PHP5 yet. Since vBGallery is not yet integrated into the rest of the sytems I'd be reluctant to say that is an 'official' answer so I'll leave that up to Chuck & Michael to validate that statement.
With that said, 'unofficially' people are doing it but they are running into the occassional glitch (
example).
Personally I'd hold off using it on PHP5 until at least Michael has a chance to install it on a PHP5 test server so that he can take a look at any issues that might arise (at the moment I don't have access to a PHP5 test server so I couldn't even help).