When you met Vanitas, he claimed to be "an average human being" who happened to inherit the Book of Vanitas. It is his stated intention to save the Vampire race by any means possible, using the Book of Vanitas not to spread Malnomen but to cure it. As such, he refers to himself as a doctor who specializes in Vampires. You knew he was lying, but went along with it so he wouldn’t mess with you. He was a mischievous man and he never takes anything serious and is impulsive. What a guy.