In the animal kingdom, there's several animals that survive solely on the blood of their prey.

Take the tick for example. A tick is an animal whos sole purpose is to attach to another animal to feed on their blood, and to reproduce.

A leech is another example. Its existance is purely in feeding on blood and reproducing.

Another is the vampire bat. Again, feeding and reproducing are its only purpose.

So far we've had an insect, an amphibian, and a mammal. A primate with this same cycle wouldn't be terribly hard to imagine.

Biologically, all a primate would need is the digestive structure of a vampire bat, a fellow mammal. This would allow him to digest blood without falling prey to blood's emitic properties. A human's digestive system only really needs to change the structure of bile that his gallbladder creates in order to change blood from a severe emitic into a digestable substance. Throw in a few extra amino acid and protien destroyers and blood breaks down into a very usable substance. It's only because blood breaks down into complex sugars and formaldeyde that humans can't use it.

Your thoughts on a primate with this digestive system please.

Well, I recall that vampire bats deal with the waste issue by secreting the ammonia out through sebacious glands, which is why a cave with vampire bats smells like a chemical plant, or a very large pool of urine. The fumes alone can knock you out. Somehow, I don't see that as being the method that a human-emulating primate would use if it intends to blend into human society. And by "blend in", I do not mean "be ignored at society's underbelly as a urine-soaked vagrant".

But woudn't that be a great cover? Nobody would suspect the piss stinking homeless guy at the park of being a vampire.

Kind of like this, eh? ^_^

Something like that, yes. You wouldn't pay attention to them because "they're beneath you". There would be little reason to hide, since nobody ever pays attention to the homeless.

Sad, but true. I suppose we would be really surprised by what sort of camouflage a predator would choose to infiltrate our cities. "Vagrant wino" probably would not be the most outlandish. Hell, if we wanted to cross this thread over to the one below it, I could point out that a butcher has ready access to large quantities of blood. So would a slaughterhouse employee. Or, if the creature in question really needed to get at human blood, you could go the medical route. Not in a blood bank, security there is tougher than you'd think, they really monitor those supplies. But in every hospital in America there are nurses or assistants drawing blood from patients and never really saying why, the patients long since conditioned to trust anyone with a white uniform and a clipboard. "Tests" for the "lab", huh? You could get away with that for awhile. Unless you stink like urine.

The vampire bat also has a specially designed stomach for digestion of blood. It has to take in a considerable amount of blood for nutrition. Blood really is not all that nutritious, it is salty and full of glucose but very litte true nutrition. The human body would have to greatly adapt if it was to take nutrition from blood. Vampires need the life energy that blood carries. "Dead" blood does not have that energy anymore and so vampires cannot feed on it. Blood banks have only "dead" blood, the life energy has long since gone from it, fresh blood carries that energy. That does not mean that the blood in blood banks cannot help the human that gets it, it only means that it cannot sustain a vampire.

First of all, blood is not so devoid of nutritional value as all that. The stuff is pretty thick with red blood cells, platelets and white blood cells, and even the plasma is kinda meaty, it's only like 85% water. Taking in a quantity of this stuff would definitely give your stomach something to do. Hell, the stuff has 600 calories per pint of loose sugar energy, not counting its protein content. Really, it's about as nutritious as well-salted beef broth, except for the ammonia and the emitic qualities, and the diseases... So it's not fair to say that the only way a vampire could survive on it is through some mysterious "energy of life".
Second, I think we're focusing too much on the vampire bat. It is mammalian, which makes it the most like us, but what if the vampire biology were more like a large tick or leech? I don't think we've really explored that possibility. If a tick had developed compressable lungs and an endoskeleton, it could grow to human size. It's not so improbable as to be a flat impossibility. It is still in the realm of science fiction, but it's not true fantasy, ya get me?
Just throwing out some ideas, someone help me out here.