I am so sick of this fight! (spoilers, maybe)

[Tancred]

Many magic items will shield one's mind from attack. A Helmet of Charm Protection, Lilarcor +3 the Two-Handed Sword, Adjatha the Drinker +2 Longsword and the Staff of the Magi +5 can all be found within Athkatla, although the Staff is very difficult to get hold of indeed. Further afield, there are the Shield of Harmony +2 or the Spear of the Unicorn +2. All will protect one's mind.

As for more generalised defences, any item that grants you magic resistance will help a little. The Priest 5th level spell Chaotic Commands can render a person immune to mind attacks, and it lasts for ages! Negative Plane Protection only lasts for a few rounds, but it is both a Druid and Cleric spell - handy against vampires.
If you have someone who has an item that grants Negative Plane Protection - and there are only two in the game, ulp - then load that character with a mind shield spell or item and use them as a human shield while the rest of the party follow on behind. With luck, all of the Vampires' attacks and charms will fail against that character, leavng you with the relatively simple task of keeping that character alive and healing him or her afterward.

Other counter-tactics: if you've been charmed, there are several ways to get around it. You've already approached the 'incapacitate' solution, to immobilise a party member while they fight, and a spell I recommend is the level 4 Wizard spell Otiluke's Resilient Sphere - it will make the affected player not only immobile but immune to anything that tries to harm him. A handy way to 'stalemate' the vamps.
Another approach is simply to lob a Dispel Magic at the affected party member, and hope your wizard's dispel level is high enough to penetrate it. Another, perhaps preferable approach is to use Remove Magic, which traditionally works only on enemies; but since the charmed character is an enemy, it will dispel his charm, reverting him to your side, while preserving any enchantments you have placed on your own party.
A useful last-ditch tactic I often use is simply charm them back! Use the Wizard spells Charm, Dire Charm or Domination to try and bring them back to your side. Usually your charm will expire after theirs, so no harm done. A magic item of note is the Ring of Human Influence, which is easily obtainable and can cast Charm Person quickly and efficiently.
Lastly, remember such abilities as Bezerk. Bezerking may be dangerous, but it will render the bezerker immune to such mind attacks; Just stay out of his or her way and everythig will be fine...

Tancred

[deathknight]

In addition to the great suggestions of chaotic commands and items that protect against charm like shield of harmony you may want to try stuff that specifically targets undead. False dawn is alright but sunray is insane on vampires. The daystar can cast that spell once per day and it is also a cleric spell. usually 1 casting of it kills alot of vampires but 2 people casting it at once can clear the room out pretty fast. I am not sure thats an option for you at your level but if you can get or already have the daystar that helps ALOT. Pierce magic might be a bit of a high level spell for you but don't forget about secret word followed by breach. if you have 2 mages have 1 cast secret word the other breach and get summoned creatures right up on tanova irst thing. without the protections hasted summoned creatures will do some nice damage. negative plane protection's duration is very short so its not as helpful as it could be.

[Avatar]

Err travel in daylight if you can't handle night life.

Fire elementals can trash Tanova.
Or cast priest spells like Call Lighting. that will kill anyone!
You can cast any priest spell outside.
Use spells like Chaotic Commands and Neg Protection!

I remember this as being one of the hardest battles since I played BG1

[Mel]

Thank you for the help. I'm finished with the vampire lair for now (I hope). But I'm sure I'll meet some others again so I'll keep all your advice in mind.
Thanks again.