Mystery. Kid goes to a hotel and solves a mystery involving a vampire and diamonds. Read late 1970s/early 1980s. Spoiler ahead.

[Michael]

I read this book in the late 70s or early 80s. It is about a kid who goes to a hotel and solves a mystery of a possible vampire at the hotel. In true Scooby Doo fashion, the vampire is a guy pretending to be a vampire to scare people away so he can find hidden diamonds from a heist that were hidden in the hoteI. I remember the end takes place on the roof and the vampire jumps off and breaks his leg and is caught. What is this one called?

[Lobstergirl, au gratin]

Sounds like this is middle grade fiction?
Cover?
Author's gender?