Natalie Angier writes a New York Times article about non-human organisms that subsist wholly or partly on the blood of other organisms. Most illuminating for my purposes is the commentary here:
Moreover, even though we rightly cherish our own blood as the indispensable elixir of our lives, it turns out that, as a foodstuff for others, it is surprisingly thin gruel. Blood is more than 95 percent water, with the rest consisting mostly of proteins, a sprinkling of sugars, minerals and other small molecules, but almost no fat. Tiny creatures can do fine on such light fare, which is why the great majority of exclusive blood eaters are arthropods…. For larger sanguivores, though, it is as much of a challenge to survive on blood as it is to acquire it.
Small wonder that wholehearted exclusive blood feeding is rare among vertebrates, and that two of the three species of vampire bats are found in such low numbers they are at risk of extinction….
The moral of the story is that blood, while rich in symbolism, is impoverished in actual food value. Therefore I suspect that most attempts to make an exclusively sanguivorous vampire biologically convincing are pretty silly insofar as they ignore the basic fact that a human-sized biped that runs solely on blood would have to be drinking it constantly [and pissing out all the watery plasma constantly, HAH!]. Much more believable to create an opportunistic sanguivore that likes blood and drinks it whenever possible, but does not get more than, say, 25% of its diet therefrom. Maybe like the LHF vampires, who are all united by their taste for human blood, but who also eat omnivorously??
P.S. Here’s Angier’s general paean to human blood, a "marvelous fluid tissue…that not only feeds us and cleans us," but also tastes delicious.