While, in reality, humanoid vampires would probably be more like vampire bats or mosquitoes. Mosquitoes, for instance, only drink blood when the females have a load of eggs to drop off at the nursery.
Why wouldn't vampires only need blood once in a while, subsisting on, say, rare meat the rest of the time? What if vampirism is a genetic trait, and there are only a few thousand in the vampire "clan," and it cannot be transmitted through the bite? They could even live in a symbiotic relationship with "bleeders" who would allow them to feed on small amounts of their blood.
So nyah! to economics and mathematics and the rest of it all.