Thai simply doesn't
have a "v" sound, just as English doesn't have a number of sounds found in languages such as Thai, the various Chinese dialects, and even European languages such as Dutch (e.g. Van Go
gh). Therefore, you don't find Thai pronouncing it as "v", just as you don't find many English speakers pronouncing Van Gogh correctly (it makes me feel like I'm making a rude noise!).
In my opinion, the "right" pronunciation is the pronunciation of the group one is chanting with. It would be silly to turn up to to a Thai Wat and create dissonance by loudly chanting a different pronunciation...
Mike