Vampires are creatures that appear in the Quake universe, though the only known specimen is found in Quake III: Arena, the character Patriot. From here, his comments and background, we can put together a basic profile for the species.
Vampires, we can estimate, are native to earth, as Patriot is obviously human, and there are no other vampires in the Quake Universe. It can be assumed that vampires are now extinct for the same reason. From Patriot's comments, it can be assumed that vampires are capable of being destroyed by silver and stakes. Assumably, vampires have immortality, a variety of superhuman powers (though this is never elaborated on) and vampirism can be passed on by some unknown method (as Patriot, at times, offers his opponents immortality). While it is obvious that Quake Vampires appear not to have any weakness to the sunlight in the Arena Eternal, it is unknown how earth's sun effects them. Despite being undead, vampires appear to have some need to breathe, as they can be drowned (though this could be an effect of running water). They also, apparently, are capable of being killed by falling.
Vampirism, seems to be a somewhat unpleasant state, as Patriot refers to his state as an 'eternal damnation' or 'torment', and expresses suicidal wishes, though at other times, he seems elated with his state, and expresses his percieved superiority over living creatures.
It also proven that Vampires can be killed through blunt force trauma, as proven in Quake III, where even a cybronically enhanced vampire, Patriot can be killed using normal weapons (Though for some reason, he seems to assume the railgun shot is a silver bullet), and seems to be equally matched with normal humans.