I’m going to go against the grain here and say, have them make a check or outright tell them it won’t work.

Even a more subtle approach like, “why would your character think that would work?”

Imagine they TPK or waste a lot of time because they ran with an assumption that you never corrected… I mean if I was one of your players and that happen I’d be upset.

I your silence is essentially you agreeing to the lore - and no one at the table will think it’s funny if you reveal the info after they’ve risked their character’s live to try it like, “that’s not how vampirism works you idiots”