For battles with Vampires, there are several methods to deal with them. If confrontation is unavoidable, use summons to tank/distract the Vampires while your party gets free hits. Mountain Bears and Fire Elementals can last quite long against Vampires.
If you have a high enough level Cleric, you can either use the Blade Barrier cheese or the Turn Undead skill. Simply cast Improved Invisibilty on your Cleric and use either skills to hit the Vampires for free. The Vampires will not notice you unless you make an agressive move or cast True Sight.
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If not a Sorcerer, then a Paladin.
