I'm sure that everyone out there who hasn't been living under a rock has noticed the rather recent obsession with Vampires™ ever since Twilight took off.
Now it seems that just about every fantasy or romance bookshelf at a store is full of Vampire Novels™.
The problem with Vampires as heroes or heroines of sorts if we go by the classical definition of “must consume blood to survive” for vampires, it leads us down the uncomfortable road of our heroes/heroines being responsible for mass murders / attempted murders / high grade assaults writ large over the years.
If we assume that a Vampire has to feed every 90 days to stay alive; that means over a 25 year period, a Vampire would need to drain a hundred people to stay alive.
Not all of these people would die, because the Vampire could only drain them partially; but a very significant fraction would die due to poor medical care at the time.
And this is just a single vampire.
If we assume there's a semi-loose 'coven' of fifteen vampires operating in NYC for forty-three years from 1900 to 1943 (when blood banks were invented), then that means that they will have drained at least 2,580 people before they could slake their thirst via blood banks.
By contrast, Harold Shipman in the UK killed only about 218 known victims with lethal injections of morphine in hospitals in modern times.
Something like this hypothetical Vampire Coven would be like the good old days where Gilles de Rais killed hundreds of peasant children in his castle between 1432 and 1440; and the Count Elizabeth Bathory, who killed hundreds of girls and women between 1602 and 1610.
Of course, the authors of these Vampire novels sort of realize just how unsavory their characters actually would be; so they try to set up “exit doors” for their characters, so they're not unsympathetic mass murderers:
In the Sookie Stackhouse novels (on which True Blood is based), the older a vampire is, the less blood they need until they don't need it anymore. Additionally, the Vampires reveal themselves to the world after a synthetic blood replacement called Tru-Blood is invented; and of course the author tries to make a LGBT / Vampire link – the phrases “God Hates Fangs” and “Coming out of the Casket” are heavily used in the novels.
In the Twilight novels, the Vampires are divided between those who drink animal blood, and those who drink human blood. Of course, Edward is one of those animal blood drinkers.
Even if the Author's vampires are “nice ones” who drink exclusively from blood banks, there's a very strong case to be made for arresting them for violating the public trust -- especially if Vampires consider Type O negative (Universal Blood Donor) blood to be like the equivalent of a very fine wine. Each time they drink that Type O Neg blood, they're denying the public that blood.
And overall, it's just better for the vampires to stay in the shadows. There are too many variables in play for vampires to ever reveal themselves to the world.
Police Departments can start to clear up their “cold case” files involving weird assaults and unexplained deaths by arresting the local vampires and sweating them until they cough up more details of what they did in the past. There is generally no statute of limitations for murder in the United States. That means the Police have “probable cause” to bring in a vampire on suspicion of murder or attempted murder simply because they exist, for one.