Ohmmmm... ...
Cheesy tactic ahead, please ignore the following unless you don't mind.
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Get two party members to drink potions of invisibilty and get them to block the doorway...
Remember to turn off party AI!
... three if necessary.
Get a ranged party member to attack the Vampires through the doorway, make sure the rest of your party are elsewhere out of sight. The Vampires will attempt to attack your ranged party member but will be blocked by the invisible wall.
They may dominate your ranged party member but it's okay, because they can't break through, and when dominated, your ranged party member can't see the wall either.
Simply wait for the domination to wear off and continue to pelt the Vampires.
Heehee, I successfully killed them all using this technique when my average party level was only 9-10.
Note: Potions of Invisibilty grants you
12 hours of invisiblity unless you make an agressive move, which is why I recommend turning off the party AI.
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If not a Sorcerer, then a Paladin.
