Emperor Cult

Then there's the idea of the deification of dead emperors. Maybe that is induction into a cabal of vampiremperors, and Nero was only notable because he refused to stay in hiding like his predecessors. That's actually cool, and I'm starting to imagine the Praetorian Guard as the living servitors of the vampires, and that's pretty freaking awesome. 

What I really dig about this is that this means it's not just a matter of different contenders for the throne marshaling different magical powers, but there are ancient magical powers who have their own interest in who wins, including dead pharaohs, vampire emperors, Parthian dragons, and who knows what else.