My point was that vampires were by definition not real - or at least, not understandable - because any time we found something real and understandable that met the definition of a vampire, we would change the definition to exclude it.

Nonsense. If there was a creature that:

• Used to be a normal living human
• Still looks human
• Has the same internal organs but none of them are functioning
• Isn't vulnerable to hemlock
• Has more strength than could plausibly attributed to humans according to our understanding of genetics
• Has teeth which extend to fangs and then retract.
• Can only be sustained by blood.
• Definitely doesn't glitter. Ever.
• Physically cannot enter people's houses due to physical restraint that seems to be only operating on the creature. Exception - can enter people's houses if invited.
• Starts behaving like the human that they used to be except with extreme sociopathic and homicidal tendencies.
• Is unaffected by getting stabbed in the chest by anything but a wooden stake. (Wooden stake kills him.)
• Burns when exposed to sunlight, holy water or religious symbols.
• Instantly turns to dust when staked, decapitated or sufficiently burnt via the aforementioned causes.

... then basically everyone would agree it was a vampire. LARPy Asimov is just being annoying when he tries to spin the question about the universe into a question about semantics.