It not exactly a contradiction in Vryloka fluff. They scarificed some of thier health as humans for unnatural vitality. In other words they traded some of a human's natural biological healing ability for the vampires ability to steal and exploit the life force of others not to mention immortality. As to being undead I figure it more an ability to emulate being undead, so they can still enjoy life, have proper emotions, have little Vryloka babies ect... This I think is what is meant by not gaining the taint of undeath. The Vryloka have the best of both worlds. Btw why Vryloka make Dampyrs make more sense of you think of Dampyrs being half Vrylokas as Vrylokas are fertile and mostly alive.
