Kind of bridging the Sci Fi an Fantasy angles, someone could probably come up with an interesting mechanic about how the blood is grown in a lab

Could be that just straight-up blood cultures grown in a vat don't have the necessary life force, but growing some genetically engineered organism that is just a living blood factory, basically a meat bag of bone marrow, circulatory system, and just enough of a nervous system to keep things going checks enough boxes for it to work.

Sort of tangential to lab-grown blood, but maybe there's a little industry preying on the families of brain-dead patients who insist on keeping them on life support and convincing them to move their loved ones mortal vessels into a blood farm when the medical bills start piling up too high. Or desperate women taking dangerous drugs and getting pregnant in hopes of having an anencephalic baby that they can sell to the vampires to make ends meet.

Maybe there was a plague that causes widespread birth defects and millions of babies are being born without any higher brain functions. The church is having an absolute conniption over this, they won't condone aborting them or taking them off of life support, but the only way many can afford to keep these human-shape meat puppets alive is by giving them over to the vampire blood farms, and they're certainly not about to condone anything having to do with those demon-spawn bloodsuckers either.