There is a good reason why Vistani PCs are not allowed, but it does involve spoiling something, so unfortunately I can't say it openly besides that it would give the player too many benefits.
[sblock]All the vistani native to Barovia are servants of Strahd, and, due to a pact, all vistani can move freely through the mists.[/sblock]
On top of which, the vistani are not native to the Forgotten Realms, but some other unnamed material plane world. They could, however, be descendants of the vistani. Perhaps some exiled member of the Barovian families interbred with the "giorgio" of Toril that had similar features to the vistani. As a result, you have a character whose blood ties are too thin to be consider a true vistani, but has physical characteristics and a similar mindset to one of them.