Depends.. Can you join the companions as a vamp?
Yes. But the Lycanthropy bit cures it and replaces it.
a nord warrior can join the mages guild and a mage or thief can join the companions.
so of course you can, as long as you keep your stage low.
Except, y'know, they're not really devoted to wiping each other out, like the Dawnguard and Vampires are?