It was a manga where the protagonist (male) was once attacked by a vampire (male), and in order to live, he had to swear allegiance towards another vampire (female). Now, he's hunting the vampire that bit him. He also works as a senior detective...
Fun fact: the author and artist who did this work did Boku Girl. https://mangakakalot.com/manga/boku_girl_2 And now I also noticed the author did some oneshots that have kinky shit.