Q. Why is there a "Guardian Angel" in the Comic Series title? These aren't "Guardian Angels", are they?
A. Vampire Guardian Angels are Guardian Angels who have been bitten by Vampires, and so they have turned into Vampires, but they are still functioning as Guardian Angels in that they still respond to prayers -- but this time prayers are a "homing device" towards their food: "praying humans".
So yes they are Guardian Angels, but they are now a hybrid of evil Vampire and regular used-to-be-holy Guardian Angel.