It's not impossible. Manga creators often come up with unique and surprising twists, so the boy next door could totally be a vampire. It could add an exciting element to the story.
The Rebirth of the Poison of the Generals manga was a limited edition manga. It told the story of the daughter of the first wife of a general. She was quiet and gentle, infatuated with Prince Ding, and went to other countries as a hostage for him. Five years later, she returned to the harem and found that there was no place for her to stay. However, other than that, the search results did not provide any more information about the reincarnated version of the manga.
It's not uncommon for such a twist in BL manga. The boy next door could totally be a vampire if the story is set up that way to add excitement and mystery.