If they have time and opportunity to set the ambush, does that mean they can place the nightblade or someone near the portal? Or have the majority of the ambush at the portal, blocking them, while just enough of the enemies are elsewhere to sort of drive them towards the portal.
If the evil magician/sorcerer has Dispel XX, you should figure out in advance whether, in your game, that will suppress the slaying properties of the weapons. The spell itself isn't clear (in RM2 at least) on whether it affects magic weapon properties, and I've seen back-and-forth in earlier forum threads on this.
One very effective tactic I've used against my players is getting enemies airborne (Fly) and then separated by more than 10'. This really reduces the value of ball attacks spells, or other spells requiring targets within 10'. Getting them airborne will also mean the paladin, nightblade, and barbarian are much less effective against those enemies. Leave someone on the ground for them to confront, but then the magician is largely responsible for taking out anyone flying.
Don't forget, either, that vampires can take mist-form, too. That's going to be harder to locate or detect, than bat or wolf form.

great suggestions. one vamp in mist form and the rest waiting to ambush as bats or flying. The party have two archers but the strongest fighter is the barbarian and hes hand-to-hand specialist.

If they have time and opportunity to set the ambush, does that mean they can place the nightblade or someone near the portal? Or have the majority of the ambush at the portal, blocking them, while just enough of the enemies are elsewhere to sort of drive them towards the portal.
If the evil magician/sorcerer has Dispel XX, you should figure out in advance whether, in your game, that will suppress the slaying properties of the weapons. The spell itself isn't clear (in RM2 at least) on whether it affects magic weapon properties, and I've seen back-and-forth in earlier forum threads on this.
One very effective tactic I've used against my players is getting enemies airborne (Fly) and then separated by more than 10'. This really reduces the value of ball attacks spells, or other spells requiring targets within 10'. Getting them airborne will also mean the paladin, nightblade, and barbarian are much less effective against those enemies. Leave someone on the ground for them to confront, but then the magician is largely responsible for taking out anyone flying.
Don't forget, either, that vampires can take mist-form, too. That's going to be harder to locate or detect, than bat or wolf form.

I have a little advantage with my application which can calculate bonuses for me based on all information. When I test Cattle I get values that are +3 more than what the book has. This is due to the +5 talent bonus. Without the book about be +2 more. This is also using Racial Hero Stats. If I don't hero stats then things are off in a worse way. Not consistent. It seems the bonuses might have been calculated using the hero stats, not the primary stats. What is really interesting is that Dog comes REALLY close. It's off by 1. Still using Hero Stats. Granted I'm not sure if the rank totals include talents or not. I am looking at all talents and modifying based on any that affect OB. Another thing Kangaroo is almost correct as well. The bonus for Martial Arts Sweeps works, but not for the kick attack. The kick is listed as 4 less than what I get in my application. Racial Hero Stats and Talents included.

One of the most powerful techniques in RM is simply the 'go invisible, sneak up, stab in back'. My PCs use it a lot, but I don't have the heart to use it often against them, as it seems a little unfair, especially if it results in a character death.

One devious idea that came to me is to have one or more of the vampires in bat form hanging from the cavern.  As you stated, they've been tracking the party and will have ample time to set an ambush.  When the party feels they are making progress, or have engaged the other NPC members, have the bats transform into vampires behind the PCs.  Flank and back attacks and a surrounded party.  This is no battle I want to be a part of!  What a challenge the PCs have as it is.

I love the idea of timing the escape through the portal.  Too late, no gets out; too soon, the evil follows them.  Fantastic.