The President's Vampire by Christopher Farnsworth

For 140 years, vampire Nathaniel Cade has been bound to protect and serve the United States of America. When an ancient evil is resurrected, the President's Vampire and his human handler, Zach Barrows, must track down its source. The prime suspect is a shady private defense contractor who may be dealing with powers far deadlier than any mortal weapon. So it's up to Cade and Zach to drag the forces of darkness into the light...