For both of them it's necessary to talk with the Statue of Molag Bal, but in the "A Cure for Vampirism" page on UESP (here: http://www.uesp.net/wiki/Morrowind:A_Cure_for_Vampirism ) they have doubts if is good to talk with it before start the quest ("what happens if you later try to become a vampire?").
Anyone know something about it? :\