As I see it, either Vampires need to have their Dracul synergy cut off or they need to be assured that an alter to Dracul will appear. As they are right now, worshipping Dracul is mandatory and there is effectively no strategy (barring obscene conversion antics) that will work otherwise. Considering vampires cannot earn piety with two of the pantheon and will automatically infuriate another two, there are already four mostly unusable alters for them. Jehora, Tikki Tooki, Pactmaker, and Taurog all have some marginal usefulness, but really the only one that shines for the Vampire is Dracul.
One possibility to deal with the Vampires mana problem: gain 1 MP every time you attack. Fits with his lifesteal motif and playstyle without being too overpowering.