Not about the Vistani, but Ravenloft itself. I wonder how it would run up against some of the base setting presumptions. Their special powers are not so much a matter of ethnicity as a combination of secret knowledge passed between members and side-effects of their regular exposure to the Mists (particularly in children born to the Vistani, since they'll be exposed to the forces in the Mists over and over again during their formative years).
Influences from a wide range of cultures can be unearthed in a Vistani camp, due to a steady trickle of recruits bringing in new sets of cultural practices and passing them on to their children, but you may have to look hard they tend to try and adopt the customs of the locals whenever they visit a Domain, and in Domains where there is a pre-existing nomadic subculture they often try to pass themselves off as another group of those people (though they know better than to try and fool an actual member of those groups). It is difficult to join them, because they have been burned many, many times in the past by those who seek to infiltrate them for dark purposes, but they do actually recruit in particular, anyone they encounter who seems to show the potential to learn the ability to manipulate the Mists they possess is considered for recruitment. The Vistani, however, are not a single, stable culture - rather, they are a cross-Domain subculture, a community of those peoples who have made a lifestyle out of nomadic travel between Domains. A (better-researched) Roma analogue might exist in Barovia, for instance, whereas a (better-researched) Bedouin analogue might exist in a Domain from the Al-Qadim setting, for instance. So, in my take on the Ravenloft setting, let's make a distinction: there are several nomadic cultures out there, but most of them stay within the bounds of a particular Domain (and obviously only the larger Domains can really accommodate them). (On top of that, whilst they absolutely match the tone and atmosphere of Barovia, in markedly different Domains they'd stick out like a sore thumb and kind of clash with the aesthetic if you go with the classic presentation of them.) It also occurs to me that, given that their whole schtick involves travelling around from Domain to Domain and the cultures of individual Domains should vary a lot, the Vistani should be a much more culturally heterogeneous bunch, both for the sake of maintaining cover when they're passing through a Domain and don't want to attract attention and because of the extensive exposure to other cultures they would pick up on. But I think it's fair to say that "magical fortunetellers who put curses on people" is not exactly a trope that most of us would be comfortable applying to our Roma analogues because goodness knows they already get enough of that shit.
I think there's a place in games for a nomadic group of people whose lifestyle in culture is inspired by the Roma, just like the lifestyles and culture of most countries in your average medieval fantasy setting are inspired by various real-world cultures. So, with Ravenloft coming back I've been thinking about ways to portray the Vistani in games which don't fall back on tired old stereotypes about Roma.