Undead are antithetical to chaos, since they lack any kind of emotion. Out of everything in the world, chaos hates undead the most. I wont go so far to say that its completely impossible though, as undead do use dark magic which ultimately derives from chaos, and I have little doubt that some chaos sorcerers resurrect undead to serve them. but generally undead and chaos are pretty diametrically opposed to eachother.
Theres also the fact that many necromancers and vampires are driven by a desire to live and/or rule forever, which is obviously conflicting with chaos trying to destroy the world.