I mean… If we are talking origins of vampires based in real fact, it is normally blamed on porphyria a blood disorder marked by a sensitivity to sunlight that most symptoms can be temporarily avoided by drinking blood. So ya they would have blood and blood flow to the genitals.
Let us also remember that originally male vampires were not described in any way as sexy or attractive, or people you would WANT to have sex with. That was specifically lady vampires. So in the infancy of vampire lore, male vampire erections were not even an issue that needed bringing up.
Why did you leave me with this question?
Im not sure they don’t have blood when they drink so much of it? Anyway how are zombies a thing if they die without brains but live only off brains?
Well I hear they’re pretty good at sucking