The phrase "...but I'm not a vampire" is part of a running gag on the Schlock Treatment podcast. It originated during fan-favorite Treatment #48, on Kolchak: The Night Stalker.
The joke usually takes the form of stating something very obviously Vampire-like (such as drinking the blood of humans to live forever), and then following with "...but I'm not a vampire."
The joke came about as the hosts of Schlock Treatment discussed the second made-for-TV movie featuring Kolchak, The Night Strangler. That film featured a character with incredible strength, who only came out at night, and drank blood to extend his life, "...but [he wasn't] a vampire."