What has happened so far? Which kingdoms are involved and on what sides?
My first thought was that the Vampire Kingdom is actually the most noble and "good" element in the war. Some, if not all, of the other "good guys" are actually looking to further their own ends in some way, possibly:
1. Prolong the wars because they gain from war.
2. Moving their armies around under the guise of war and suddenly attacking an ally. They are secretly allied with the "bad guys."
3. Vampire Kingdom leaders slay several of the other "good guy" leaders for unknown reasons: they weren't "good guys" at all, but devils in disguise.
Those are the thoughts off the top of my head; if I had more info to work with, I could come up with more ideas.
/I never ran Greyhawk.