Vampiri are like the mortal version of vampires. They're almost always male, in that no documented record of a female has been discovered, which means they mate with female humans. The vampiri is born as one, clawing and chewing its way from its host's body and killing her in the process of its birthing. The father usually does not have a lifestyle that is conducive to raising a child and either abandons it or puts it up for adoption, if he even knows about the child. One can go its whole life without knowing he's any different than anyone else. He'll have cravings he can't explain, most likely starting in his teens. If he doesn't discover his penchant for blood he lives a normal human lifespan. The blood is the life, and it prolongs life and youth for the vampiri just as it does for their monstrous counterparts.
Differences from Vampires
Unlike vampires, the vampiri do not have retractable fangs. They are born with canine eyeteeth that fall out within a couple days of birth. Their canines, when they grow in, are longer and sharper, more pronounced than those of humans, but still are not sharp enough to tear skin without making a mess. Most vampiri file their canines to a sharper edge and use them in combination with razors or knives to feed. Nor do they possess that mysterious gift that grants vampire victims pleasure and passiveness in the feeding process.
Yet another difference is in the way vampiri procreate. They can not embrace or be embraced. They must be born, and if one attempts to embrace a vampir into their clan, the vampiri will simply die as the two bloods battle each other.
Vampires generally despise the 'mongrels' but will not combat them unless strongly provoked, though they are not protected from the hunters.
Interaction with VX
Since a vampire is the result of an infection with the VX Virus one can think of the vampiri as a creature which is at the same time immune and a carrier to VX, though due to the unique nature of the VX Virus, it is a mutated variation that cannot spread beyond the host it is born into.
References
Lost Souls - Poppy Z. Brite
Credit
Credit for information goes to Spook.