This has bugged me from day one. I enjoy playing Clerics. And I usually make them Clerics of Kelemvor. When first meeting Astarion, a Cleric of Kelemvor "should" immediately recognize him as an Undead, and attack him out-right. If not, once he is caught trying to feed on the Cleric, it "should" be an immediate combat to the death. After all, Kelemvor is "Fair but distant, Kelemvor guides the dead to their appropriate plane in the afterlife. His clergy provides last rites across FaerÃ»n, but also destroy Undead that have escaped Kelemvor's judgement".