As the title says really. My party are currently sneaking into a keep infested with vampires via an abandoned escape route. One of the suggestions I've heard on why the tunnel would be abandoned is that some thing lives in there and the vampires decided to make an uneasy truce with it, rather than try and exterminate it themselves (after all, it makes for a good guard dog whatever it is).
So, has there ever been any indication in D&D (preferably 5e, but any edition will do) that vampires ally themselves with other monsters? If so, what kind of monsters? i.e Beholders, Demons, Abberrations in general, etc.