??? Am I missing something?
Snape had them write an essay about werewolves, which is how Hermione figured out Lupin. It may be my faulty memory, but I don't recall an essay about vampires.
Lupin “retaliated” with a vampire essay.
I know some people thought Snape was a vampire but I never believed it... of course I don’t think he was an animagus either, I think Rowling meant he was flying without a broom, but in human form, like Voldemort was. Not that he had turned into a bat.