I mean, you've got a few options.
One is to just raze the fiends. As the Dwarfs you can't occupy Sylvania anyways, razing is the only permanent solution. The humans will _probably_ leave that land alone (though there's a slim chance they'll colonize it, let a rebellion happen, and suddenly the Vamps are back!).
The second is to take the peace deal. It's a minor diplomatic penalty at best for signing peace - in fact, I don't believe there's any penalty for making peace, so the only penalty is the tiny ongoing one for "deals with Vampire Counts" which will be, like, -1 relations or something irrelevant. That said, I would point out that, if they're actually offering 6k, that means they've still got a pretty high economic strength, so you might want to deal a little more damage to them before you peace out and let them rebuild their army!
Also, fun fact: being in a military alliance obligates you to join any wars your allies start, but for some crazy reason does not require you to join any war your allies are already in. Assuming you won't suffer a trustworthiness penalty for declaring peace, you're free to take the offer, and then you may reject any request from your 'allies' to rejoin the war without even a relationship penalty.
The third, which only cool people with high self-confidence will do, is vassalize them. The upside is that you get your very own Vampiric minions to direct against whatever foes you see fit. The downside is that your relations will go down much more rapidly than if you'd just peaced them out, not to mention that Manny is a backstabbing schemer and will probably betray you before you have a chance to do anything with him. But hey, vassalizing an undead overlord is far more fun than wiping them out or aborting the war, so why not?