This could be "And Not Quite Human"
This story is by Joe Hensley, and was published in 1953. It was included in the 1994 sci-fi vampire anthology Tomorrow Sucks, which may be where you read it.
Aliens destroy life on Earth, but are surprised to see that there are a few survivors:
"How are the Earth specimens, Doctor?" the older one asked, his
voice indifferent. He touched his splendid purple pants, straightening
the already precise creases.
"They stare at the walls, Captain.
They do not eat what we give them. They seem to look through the
guards, say very little and use their bodies feebly. I do not think
that all of them will live through the trip."
"They are weak. It
only shows the laboratories are wrong. Our people are not related to
them—despite the similarity in appearance. No, we are cast in a
stronger mold than that." He drummed his desk with impatient fingers.
"Well—we can't let them die. Force-feed them if necessary. Our
scientists demand specimens; we are lucky that some of them lived
through the attack. I don't see now it was possible—it was such a
It is later revealed that the supposed human survivors are in fact vampires:
"It was a wonderful attack, Captain," the old man said softly. The
shadows nodded as they formed and faded. "Nothing human could have
lived through it—nothing human did. Some of us were deep underground
where they'd buried us long ago—the stakes through our hearts—they
knew how to deal with us. But your fire burned the stakes away."