Dance in the Vampire Bund, Itsuka Tenma no Kuro Usagi Recommendations
Both have to do with a vampire girl who has a protector. Both guys get amnesia for one reason or another, and then they eventually remember their girls and start protecting them.
Main girl character is a vampire. Main guy had amnesia and forgets the girl, who he "serves". They're in love with each other. The main guy both has some power.