I have not found in other threads of the forum an extensive resource on Half-Vistani despite some useful information are available in game, when selecting the subrace at creation (like for instance the choice of the tribe). These are anyway resources which are common for all Half-Vistani and should kept in mind when playing one. The resources are taken from "Domains of Dread" pages 240-242. I will summarize when I can and quote when I must.
Vistani are not a playable race, as pureblood, but the Half-Vistani can be played. Their name is
Giomorgo, who are born by a parent who is a Vistana and one who is human. A very rare version of Half-Vistani is the
Giamarga, that is a half breed of a Vistana and an elf. There are not giomorgo communities, but, as other half-breeds (such as half-elves), these are folks who have some difficulty to be accepted by any of the groups of their parents. Giomorgos, in fact, may be accepted by Vistani, but the reaction of them could easily be the same xenophobic one of a typical Barovian.
Giomorgo resemble physically the pureblood Vistani, keeping the same dark complexion, dark hair, dark eyes (from black to dark brown). Their features tend to be angular. Giomorgo can prefer the style of their Vistani parents or that of the human parents, but whereas humans can too easily be deceived by their dresses and behaviour, Vistani will never be fooled by a giomorgo: that is, a giomorgo will be confused for a pure Vistana by a human, but she will never cheat a Vistana; same it goes for a giomorgo who tries to pretend being fully human, humans could even consider them as such, but Vistani would see through the "disguise". The fear of the revelation of their true nature is a sort of paranoia which should be felt by any giomorgo character: some could hide it better than others but never defeat it completely. Nevertheless, on the other hand, they won't ever deny fully their own mingled identity. Paranoia on a side, pride on another.
The giomorgo tend to be loners, to have a good skill to find allies and many acquaintances, but never fully trusting anyone. Their embittered souls basically do not allow them to be more compassionate than they are, and they do not even pretend to be.
They have some special powers. Some of them are already described when selecting the subrace at character creation, but there are some things which are not "mechanical" and need to be RPed out.
Giomorgo can identify easily plants and animals as a druid and determine which water is safe to drink. The relation with nature does not end here because they can easily make campfires too. Giomorgo are familiar with tralaks, the "secret" glyphs used by Vistani, with very favourable rolls (after a successful INT check, only 1 on 20 is a fail to really understand what's the meaning). It is worth to point out that giomorgo are
not supposed to know Patterna, but only this enhanced confidence with tralaks.
The final thing I would like to point out is the
Lunatio, the "curse" of giomorgo (Vistani are supposed to be more resistant to this curse). It happens in the THREE nights of the month when the full moon is shining. The giomorgo, during those nights, will be wholly unable to sleep, thus being, supposedly, unable to heal non magically and to memorize new spells. Supposedly the character should suffer from -2 penalty on all the saves, if somehow it is not technically doable, consider if you play a giomorgo you feel basically weaker. The Lunatio has, furthermore, a maddening effect: on each of the three nights it should be rolled a madness check. If the first fails, the giomorgo will try to run wild under the stars; if a second time it fails, the giomorgo becomes aggressive and violent; if a third time it is failed, the character is completely insane until the moon phase changes again.
On the Lunatio, of course there cannot be a full availability of a DM at all the times to supervise this, or even "enforce" this. But it is a vital part of the subrace which should be roleplayed autonomously such as, to compare to an example, the moon phases for a werecreature or the approaching of the dawn for a vampire. And it could give way also to some form of creative RP

Cheers for now.
[EDIT] Half-Vistani is GIOMORGO and not GIOGOTO as sometimes I have seen in game. Giogoto is a special role within Vistani tribes which is given to some loyal, very friendly, non-Vistana. A giomorgo CAN be also giogoto but it is not the same thing, and it may well be that a giomorgo is fully despised by the Vistana tribe of their parent.