The Dead War was never supposed to be a trilogy. Hell the first book was only supposed to be a stand alone story. As a matter of fact it wasn't even called The Dead War it was titled Scout: Survive The Night. It actually ended after Sgt. Richards reached The Rock and with his handing over of the vital information he carried to General Holloway. Some of who actually brought that book may have noticed that at one time Sgt. Richards wasn't even named that he was Sgt. Richardson.
After the book sold more copies than I ever expected I decided to write another and to make it at least a trilogy. In my mind this meant having to better explain the cause of the dead walking and adding to the first book. Scout: Survive The Night became The Dead War Series Book One. A prologue was added to explain how the Beserker Virus was released and the book ended with the first battle of The Rock.
Now in planning out the next two books I found myself running into a problem. How would I explain the actions of Duncan and Mitchell in the prologue of book one. I didn't think anyone would really release a virus that would wipe out mankind just for money and wanted a better reason than that. I also did not want to go the full blown conspiracy route that would have involved creating an entire mythology for The Dead War world that maybe I couldn't keep track of.
I damned sure didn't want to go the well worn ( and tired ) route of the evil corporation such as Umbrella Corporation, Weyland Industries, or OmniCorp so I had to think of someone or something with a reason to release the virus.
After some thought I came up with the idea of vampires releasing the virus in an attempt to "thin the herd" and weaken mankind before an all out assault would be unleashed. This made since to me because humans would most certainly outnumber vampires and any attempt to just attack outright would most certainly fail.
So there you have it. That's why vampires appear in The Dead War Trilogy.
George Cook, The Dead War Series