Image credit: themoodyfashionista.blogspot.com.au Image Description: a container of red tictacs marked "Vampire Vitamins". End description.
I know among humans that word means something completely different, but when a vampire talks about a vegetarian we're talking about someone who does not feed on human beings.
Some of them don't drink at all. Vampires who consume other forms of energy can do it with or without humans.
That's a fair question. Not even human vegetarians are immune to it.
Between staying off hunters' radars and not killing humans, the benefits of being a vampire vegetarian are pretty obvious. I mean, I'm not one. Human blood tastes good. You guys are delicious. But many of my friends are.
Many vampire vegetarians simply don't want to harm humans and/or don't trust themselves to feed on them at all. I mean, that must have occured to you, right? Some vampires have *ahem* weak stomachs for causing mayhem. Many of us are peace-loving people who see humans as more than a food source. There are plenty of vampires who strictly abstain from drinking human blood because they don't want to cause human suffering.
Others prefer to keep a low profile. Human victims draw far more attention than other feeding methods and to some vampires you guys just aren't worth the hassle. If feeding on humans and leaving a bunch of empty juiceboxes around doesn't bring in the human cops, it will certainly bring in the hunters. Feeding on humans properly is a lot of work. You have to keep bloodstains out of your clothes, hide and dispose of bodies, select victims carefully, work work work work work work work.
Finally, there are a tiny set of circumstances in which vampires are forbidden by law from feeding on humans. In tight-knit, secret communities which house supernatural beings of many kinds as well as humans, everything has to be above board. If you live in one of these tiny communities, you give up on feeding on humans. It's not possible because you have to be able to trust those around you so they need to trust you too. In these villages, everyone knows everyone. Everyone will know what you've done. This is what keeps everyone, humans, vampires, werewolves, anything, in line.