No special weapons are needed. I would guess a higher level Freeze spell would work, but I never played a mage, and don't know what their resistances are, etc. I cleared the majority of Rivertown before heading to Verdistis, so didn't have any problem with the vampire there. For the one in the dark cave I don't recall what level I was when I first got there (cleared around Aleroth, the farmlands and Ars Magicana), but I had to leave and return later after I had leveled a bit (and found better equipment).
In the dark cave, if you get in behind the coffin as far as possible before opening it, that may give you an advantage. Someone mentioned using Deadly Disks to take out the Verdistis vampire before it could get to them. Meterorstrike has a bit of a stunning effect, and can be cast quite quickly (and if you can get off a cast of the survivor's Curse spell first, it would be more effective). Mostly you just need to be a higher level, though.
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