The Case of the Visiting Vampire by Drew Stevenson, 1988.
This is the second book in the series about J. Huntley English, an extremely intelligent boy who also believes in real-life monsters and is determined to become a monster hunter so that he can find some. Along with his best friend, Raymond (who narrates the books), and Verna, a bossy girl who goes to school with Raymond, he investigates possible monsters around their small Pennsylvania town. The stories are humorous mysteries, and in a kind of Scooby-Doo fashion, there are other explanations besides monsters for the mysterious things that happen.
In this story, the Big Lake Theater in Barkley, PA, is putting on a play called The Count of Castle Dracula, and a visiting actor from Romania working at a local college is playing the part of the vampire. When Verna gets a part in the play, she becomes convinced that the actor, Bela Mezgar, is a real vampire. He behaves strangely and is never seen out during the daytime. Then, Verna overhears him talking on the phone to someone about his coffin and saying that “one heart must die so the other may live.” She tells Raymond and Huntley what she heard, and the monster hunter eagerly begins his investigation. However, a mysterious stranger also seems to be showing an unusual interest in the actor.
This book was written before the end of the Cold War, when Romania was still a communist country. Although things have changed since then, I don’t think that modern kids would have too much trouble understanding the situation. It’s a fun mystery for elementary school kids.