I believe I read this in the early '90s. (So before I developed my allergy to vampire stories.) I don't recall if this is a short-story or part of a larger work; there aren't any plot details here so it's obviously not complete as it stands.
The protagonist is a vampire and is reflecting on the popular mythology that to kill a vampire you need to drive a wooden stake through its heart. (I don't recall if this is an internal monologue or he's actually explaining it to someone. I'm pretty sure the vampire is male, though.)
The trick is that myth was started by vampires specifically because they can learn (with practice) to almost instantaneously absorb the wood, so their heart can just skip a beat and continue on. But it only works with organic matter, so an iron spike would successfully kill them. If I'm remembering correctly, he first practiced with driving small wooden spikes into his arm, where they wouldn't kill him if he was too slow.
I can't remember anything about the setting, except I think it was modern-ish (approximately 20th century) instead of being mediaeval or completely fantastical. Artificial lights, paved streets, that kind of thing.