The prose is often so vague as to be almost useless, and frequently contradicts itself. While this is sometimes clearly just a mistake (the text sometimes flips what it's saying from sentence to sentence), at a higher level it's deliberate. Instead of answering questions or explaining how the Vistani work, the Guide repeatedly states that would take away the mystery, so the DM should make something up. But while that's true to some degree, there isn't any good advice for filling in the blanks, either. Much of the confusion comes from wanting to play both sides of the fence -- on the one hand, the book makes blanket statements about how they're all opportunistic amoral thieves, but on the other hand, it tries to present them as unjustly persecuted. On the one hand, they're horribly victimized by 0-level peasants, but on the other, it states the reason they can sleep in the open is because they're so powerful that monsters of Ravenloft avoid them.

The defined parts do explain some shared customs and abilities, and cover the unique cultural, magical, and mechanical characteristics of the different groups of Vistani (who are broken into three broad "tasques" or nations, which are further sub-divided into a total of seven tribes), but they're generally not very interesting -- one group likes animals, another are known as blacksmiths, and yet another are particularly charming.

They're quite literally magical gypsies, with powers and limitations that enforce that role (all their women can use the evil eye, they can all curse, if they stop in one place too long they suffer "static burn" and stop being Vistani, etc.). And it's not just a few powers, or a list of preferences; there's a whole suite of minor abilities where the Vistani are simply better than anyone else, just because it's supposed to make them "mysterious". And their major abilities are almost all explicitly plot-based -- the DM is supposed to use fiat to force them into a specific role, and prevent the PCs (or anyone else) from stopping it. Which means they're not people, they're just plot devices.