You need a +1 weapon at least to harm a vampire (I think). since by the time I fight some of them I have at least a +2 or +3 handy from the adventurer's mart.
Also turn undead is really lethal to them and so is sunray.
(Sunray can inflict a potential 1000+ damage to them!! - No Kiddin'!)
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Greetings mortals, my name is Wah, Arch Lich Wah.