There is a big problem, that they drink too much - to have long time working co-existence, they need too much people in relatively small area to feed on
family of vampires (two adults, two childs) needs either to kill and dry adult human every week (and the human population have to grow fast for that - count 1 child/year per 2 adults for 1/3 of their life, so population like 450 people could barely feed one vampire family, and somehow survive, if no other deaths are involved), or can they prey alive to regenerate (witch results to like 120 adults and 60 childs to be rotated - still pretty big village for ancient times)
And that is the minimum just to survive. So Vampires needs to be pretty teritorial, slow growing population with only few families in big area, else they would extinct from killing its food too fast.
Also humans are good cooperators and the vampires would be hunted terribly, if they misuse humans so much (and vampires are younger than human, as they are specialised on human as food, so they would be at big disadvatage at first, until they fully extend they powers, get higher in numbers and establish some kind of stable manageble situation)
There are two possible solutions for that - first the vampires would enslave humans as food and took care about them as we do for cows, pigs etc. (but it is totally other world), or second ther will be just few vampires, highly mobile and they would have something to offer too, so humans would not try to destroy them all at all cost.
And that would be possible, if vampires would use something like leeches, but offer a lot more - anesthetic and anticoagulant (for obvious reason), but also some drugs to paralyse the prey (for the time of drinking), other drugs to provide happiness and pleasure to the prey (so the prey would not like to retaliate, but may be willing to provide blood for pleasure even next time), some good medicine (to prevent prey from death and for faster recovery - it could also provide some kind of resistance to other parasites, illness and other common case of premature death, as well as make it heal faster to not lose much blood and recover it faster - which would help also with other medical problems so wounded/ill people would seek for be used by vapires to survive the original wound/illness and recover faster).
Also even when digestive of vampires is superefective, there can be some residuals, that would vampires dispose back to their prey (and that would help recovery as mentioned before).
If vampires are specialized in humans (and magic is involved), they may even not digest the blood at all, but just mix it with their blood directly, spend all food from it as well as some other parts and then return the blood to their next prey (so using humans as "external digestive system" for themself).
This way the prey would recover even faster (but be really hungry), as it would not lose so much of blood, only part of its blood would be fitered out of usable food and quality and such, but overall blood presure (and volume) would be nearly the same, maybe even some residual from vampire extra speed/strenght/enhanced sences would be there and the prey would be for few days faster and more powerfull itself, so be able find food and recover and have more success in human society, as well as be more fertile (so successuful prey would have more childs more ready to be prey too and "marked" people would be local heroes, so be prey would be kind of blessing (even at high personal cost))
So the vampires may be not killing demons, but dangerous symbionts of human race (at least in some cultures)