Those who have died, been sent to the Paths of the Dead, and for reasons known only to the Gods, been allowed to return to the land of the living. This is a different thing entirely from reincarnation.
While it is known that Sethra Lavode is a vampire, it is conceivable that there are other forms of undead on Dragaera. Vlad Taltos hints at this in his conversation with Savn.
A vampire, of which we have one example to generalize from, eats little, and only needs a small amount of blood to continue to exist.