In a Ravenloft adventure, the players are generally confined to a region from which they must escape by solving one or more problems. The Vistani help the referee ("Dungeon Master") with the mechanics, especially in allowing players to interact and negotiate with strangers from a different culture. Their abilities have been developed accordingly.
Vistani have some control of the Mists of Ravenloft, which divide realms. Players generally need help from the Vistani to travel realiably from one open realm to another.
Vistani possess a powerful wanderlust, and cannot stay within a mile of a given point for more than a week before suffering a condition called "Static Burn," where the Vistani is afflicted with an illness resembling the flu or a fever. When this occurs, the Vistani has between 2 and 7 days to cross the necessary ground or permanently lose all of their mystical powers, becoming an outcast known as a mortu, which translates roughly as "Living Dead." This mechanic ensures that contact with helpful Vistani will occur only when the Dungeon Master wishes.
Vistani possess powerful, innate magical abilities that recall Romani as imagined in popular culture. "Curse Potency" is self-explanatory: curses in Ravenloft have great power, and the curses of Vistani are more likely to come true. "The Evil Eye" is the ability to mystically blight another person's luck, whilst "the Sight" is powerful divination magic, allowing them to predict the future. The Sight is only allowed to exist amongst Vistani women; it does occur in Vistani men, but any male babe detected to have the Sight is slain.
Vistani society has ranks: Raunie is the title of the female Vistani in the camp with the most power in the Sight. A Darkling is a Vistani criminal who has been cast out from the tribe; they hate and are hated by Vistani. A Dukkar is a male Vistani born with the Sight who has grown to adulthood. Such a being will inevitably bring great suffering to the Vistani. The only Dukkar alive is a half-fiend who rules the domain of Invidia (but is not the darklord for that domain). The Vistani who wrote the prophecy foretelling the Grand Conjunction was inferred to be a dukkar.
The fictional character Rudolph van Richten, a famous Monster Hunter in Ravenloft and author of a series of guides to hunting and slaying various monsters, had a great enmity for the Vistani for most of his monster-hunting career, but his opinion improved during his penning of Van Richten's Guide To The Vistani, during which he befriended a mortu.