Hm, well, they'd be Buffy-type vamps, who do eat and can breathe if they want to (ref. Spike's smoking, and dunking weetabix in his mug of blood, plus the actual drinking of blood), but some of that at least could be put down to the demon, possibly? There's also the complication that this story would be taking place 500 years from now, so maybe science will be more advanced, then?

*ponders this more*