I cant remember who brought up vampires, but thats an interesting twist.
Vampires and zombies both tend to prey on humans.
With the exception of the 04(?) DOTD, zombies were extremely slow. And with the exception of Land of the Dead, they also had little to no intelligence. They pretty much walk around, moan and groan, and eat dead or dying people.
When compared to humans, Vampires seem to be very educated, of course, they have more time to learn. They also have more strength, speed, and agility than humans. While they do have weaknesses, they arent anything most people would carry on a regular basis, at night. They, of course, lay low during the day because of the sunlight.
Lets say that Vampires and Zombies do in fact exist. Two predators with a common source of food. Being the most intelligent, Vampires know that they cant survive without us to feed on. Zombies dont, they eat whoever they can. So, to keep from starving, the vampires would have keep the zombie population in check, they might be doing that right now...
In the event of an outbreak, like in DOTD, the vampires will organize against the zombies. They thought that the feds and the population could handle it, however Katrina proved otherwise, and they arent gonna take any chances. As a matter of fact, they already have plans A, B, C, and D on the table. Plan D is "improvise," something most people use for plan B or C (its normally my plan C).
So, now you have to ask yourself, is the enemy of my enemy, and myself, my friend?