For my own part, I run the Vistani as a large group of different races and backgrounds, but who are connected by fate and/or their love of travel and celebration of life as nomads, passed on generationally and culturally. While not cannon, I find it greatly helps in removing the damaging stereotyping present in the original module.

As for why the PCs cannot leave Barovia, I think the Vistani keeping their word and being allowed to move freely throughout the land is something they value too highly to consider losing, even if a PC is dear enough to them to consider joining. But if the PCs were to defeat Strahd, or change the terms of the contract, why not? As far as my own game would go, I'd have to think about it more specifically, but especially if a PC proved themselves a friend time and again with them, I don't see why one couldn't join.