I do presentations to reading groups at libraries and colleges, and in the course of preparing for them, do a bit of research on the types of books available in certain genres. It's surprising to find that whatever one might think of in the line of mystery, it's probably been done.
Vampire detectives? We've got 'em. Sleuths who've been gelded for one reason or another? Yup. Females masquerading as males, ex-soldiers suffering from delusions, police detectives with amnesia, and of course, various folk who receive messages from the dead. (I often wonder how they manage when driving past cemeteries!)
So how does a writer fit in? I recommend two things: great characters and clever plots. Even if a device has been "done," your work can find a place in publishing with good writing and unique facets. Maybe your vampire has a sister who wants to come out of the casket. Maybe your feisty Revolutionary War heroine has an affair with Ben Franklin. If the characters are well-drawn and the plot is enticing, it doesn't matter how many vampires are out there. You work will rise like a bat out of... Well, it will. Eventually.