Since I doubt vegetarianism is a thing in Skyrim, everyone else has to actually kill to survive, whereas vampires don't need to.

Those vampires disinclined to cause mayhem probably keep it quiet in the same way that Sybil does. The ones we fight in the wilderness are effectively bandits, which I suspect might be their own bizarre little subspecies of Nords that doesn't entirely comprehend the idea that murder isn't actually necessary to make money.