Vistani

There is far too much information I have gleaned about them to put entirely into bullet points. I heartily recommend having a look at the book "Van Richten's Guide to the Vistani" if you intend for them to play a large role in your campaign. There's a lot of really cool information in there. In fact, you can get a lot of cool fluff about a lot of Ravenloft beasties from the Van Richten's Guides. Here's just a handful of highlights: (Source)

Vistani slang

The Vistani name for anyone not Vistani is a giorgio. A giorgio who has performed some great service to the Vistani may be redubbed a giogoto through a blood rite...a Vistani won't cheat a giogoto, and may offer even more information than they asked/paid for. And, a giogoto is always welcome to spend the night in a Vistani camp. (Source)

Physical distinctives

Aging

Vistani don't age normally...and it's uncertain if they can actually die of old age. The theory goes that they grow from childhood at whatever pace suits them, get to a degree of physical maturity that they like, and stop. There are 'old' Vistani, because they have great respect for their elders. But a Vistana's appearance is no indicator of their age...and most eventually stop counting "giorgio years" and will just make some cryptic remark about the nature of time and how brief mortal spans are if you ask them their age. (This means that Ezmerelda d'Avenir--whom we know to only be in her 20s or so--is extremely young for a Vistana). (Source)

Need to travel

Vistani must travel. If a Vistana stays within a 1-mile area for more than a week, they rapidly grow ill and develop a nasty fever. When the fever breaks...they are no longer Vistani. They have lost all of their special abilities--never to regain them, and begin to age as a human would. The Vistani term for such individuals is 'Mortu,' and they are shunned from Vistani society. (Source)

Leadership: Raunie

Vistani caravans are always led by a female Seer, called the Raunie. The fact that the group near Vallaki is led by two brothers is a sign that something is wrong (they are fully allied with the Dark Wizard). (Source)

A Vistani seer is utterly incapable of seeing her own future. (Source)

Allegiance to Dark Wizard

Typical Vistani will provide information to Strahd whenever he asks, as part of their agreement that gives them passage through his lands...but that's the end of their loyalty. (Source)

Relationship to Barovia

Vistani can actually exert control over the Mists of Barovia. They can call up the mists around them pretty much whenever they want, and have the mists whisk them away to pretty much anywhere they want to go. (Source)

There are three completely discrete 'nations' of Vistani that are quite different, called Tasques...each one is further divided into Tribes...which is then split into discrete caravans. There's a lot of cool information out there about the Tasques and Tribes. (Source)

Language

They have their own language called the Patterna which is a cobbled together mess of gods-only-know how many different languages. It is extremely information dense, though...they can communicate very complicated ideas in only a few words. They also use a series of marks similar to Hobo Marks (called Tralaks) for long-lasting written communication...usually as tips or advice to other caravans that pass that way. (Source)

Survival skills

They are uncannily good at starting fires. Give a Vistana a pile of wet seaweed in a thunderstorm, and he'll have a fire going for you shortly. They are also uncannily good at destroying evidence of their camps after they leave. (Source)

Key customs

Evenings with the Vistani always play basically out the same way. First a one-woman dance called the Prastonata that is always improvised (the musicians follow the dancer, instead of the other way around) and is meant to reflect the mood and thoughts of the caravan at large. Then story-time (the Doroq) where they recount their legends and encourage visitors to share stories of their own (they don't particularly care if the story is true, false, or just a good joke...it's all good). (Source)