The Vampire isn't that hard and you don't have to have a bunch of spell casters to beat him. Last night my group beat him on normal and then again on hard. The group consisted of 2 fighters, 1ranger/rogue, 1 Paladin, 1 cleric (me) and one wizard. We are all 8th and 9th level.
The first time on normal, we went in and hit hime with MM and searing light and within about 2 minutes he was done.
The second time was a lot tougher until i realized something, its so simple and its staring us all right in the face, you can negate his regen. I'll say it again "YOU CAN NEGATE HIS REGEN ABILITY" Now thats not going to drop his a/c you will still need at least 2 casters a cleric and a wizard. But it can be done. When i figured this out last night we failed to kill the vamp on hard 3 times, then the light bulb went off and we marched right back in and dropped him in 20 seconds and that was on hard. The beauty was I only used 3 charges from my searing light wand.
Here's how to negate the Regen:
1st you need a cleric, and your going to use the spells that clerics use the most, thats right heals. But first you need to go by a few potions of Improved Ardor V (this grants a 50% increase to you 5th level and lower heal spels by 50%) The potion will only last 30 seconds so here's what you do.
Set up a shortcut bar just for this fight.
Put your searing lite wand in slot 1 put you searing light spell in slot2 put your cure critical in slot 3 cure moderate in slot 4 cure serious in slot 5 protection from eveil in slot 6 protectection from fire in slot 7 a second wand of searing light in slot 8 (I doubt you'll need it) your potion of improved brillance III in slot 9 and IMproved ardo V in slot 0
Your wizard or mage will spam MM only, let the fighters knwo that there willbe no healing during this fight. IF you stop to heal them you will fail to put down the vampire. You alson need to tell your fighters that the wolves are top priority and they need to keep them off the cleric and the wizard.
Just before you go in cast pro fire and pro eveil on the entire party, pro eveil will help your caster from being dominated, you way also want to mem remove paralisis as well, so if your mage does get dominated you can remove it.
Have 1 fighter run in and draw aggro on the vamp, Have the wizard hold off on MM until the vampire has lost 5 10 10% of his HP. This will happen with your 1st cast or 2 of searing light.
Walk into the room and you will drink your brilliance potion first, ador v second spell cast searing light, heal, heal , heal, heal, (on the vampire), wand cast searing light, as soon as you use your wand tell the mage to let loose with MM, and you repeat the sequence. If he goes bats he'll come back with some more health but usually not enough if you have taken hits from flame strike you will be in your casting sequence and since the target has disappeared you will heal yourself. Buy the second time he appears you should have dropped him.
IF you set your shortcut bar according your button sequence would look like this" 9,0,2,3,4,5,1,2,3,4,5,9,0,1,2,3,4,5 etc, use the wand if your cool down on searing light hasn't reset yet. When i ran this last night i was able to get every spell off and never had to wait for a cool down. As a cleric you will drop the vampires hp buy about 25% and stopped him from regening. Since your taking the adorV potion you are doing damge equal to his heals and all the mages MM are ripping him up. When we ran this on hard last night the Wizzard hadn't even used half of his spell points and I only used about 300 and that was including the buffs for the party prefight. OH you will also want to cast owls wisdom on yourself to helpo maximize your spellpower.