If vampires existed, they'd expand upon their must-be-invited rule further. They'd create reforms in law and start practicing euthanasia.
Eventually they'd have access to more blood than they could ever drink (which, along with therapeutic analysis by said vampires, would add to the vetting process), doing advanced blood analysis on every patient and donating the best blood (which has no purpose for undead) to blood banks/hospitals/research.
That and I'm sure they'd also do stuff like figuring out how to turn less palatable (bagged) blood into some kind of beer or other foodstuff product for vampires. Or maybe just filtration/purification.
Well, except I'm sure we wouldn't have anything like this in the US. The metaphorical vampires wouldn't want to allow an exit. Vampires would instead stay in the shadows and be an unchecked medical problem and additional violence crisis even if prevention/treatment/cure options existed. And I'm sure at least one kid wearing plastic fangs* and a cape would be killed by someone acquitted, because "you can never be too safe".
*= also anyone who hasn't had their pointy cuspid teeth filed down by a dentist. Probably someone would be killed at an office party or something.