Dolivar was the brother of Kieron, and initially a Dragonlord. However, for reasons that are not clear, he betrayed the house of Dragon and was expelled from the tribe and subsequently founded the Jhereg. See Jhereg.
Vlad Taltos is the reincarnation of Dolivar, according to Aliera e'Kieron. Kieron appeared to recognize Vlad (with about the right amount of joy for a traitorous brother) in the Paths of the Dead as well, adding credence to this idea. Vlad's friendship with Kiera, elaborated in Orca, also seem to confirm this.
Sethra Lavode had a fondness for Dolivar, which may have led her to spare his life at one point. She has commented on several occasions about traits of Dolivar's that are also present in Vlad; which may help to explain the basis for her friendship with Vlad.