A kid once asked to pet my dog and I said no. Kid then proceeded to tell me my dog looked evil. I was enraged. She is a dog, she can't be evil, and she's absolutely terrified of strangers but especially kids.
Usually when I have to deny people petting this dog, I'll offer to let them pet the other dog who's normal, but fuck that kid, you don't get to pet any dog today.
In BTVS they can. There's a vampire who's cursed with having a soul and is therefore unwilling to hurt humans, so he just toddles down to the butcher and gets blood from them.
Soulless vampires either like the hunt or just prefer the taste of human blood. For instance, some vampires just raid blood banks instead of hunting humans.