You cannot base your character's motives just on game mechanics alone. Dawnguard took away stage 4 hostility for a number of reasons, and I do believe it had to do with the massive uproar on the official Bethesda forums regarding vampire lore. I know, since 99% of my topics and posts were all about werewolves&vampires.
In vanilla Skyrim, the gameplay mechanics were just like Oblivion. Feed and you weren't recognized. Don't feed and you're known for a vampire. The only exception is that people attacked you. Lore wise, only the vampires in Cyrodiil have the ability to blend in because of a pact they made with Clavicus Vile. Which explains why they can walk among mortals and even become politicians! Yikes!
In Skyrim, it's sort of contradictory since the higher leveled vampire NPC's are dubbed "Volkihar", which are not the same clan as the Cyrodiilic ones. Additionally, Movarth Piquine is in the game and he was bitten by a Cyrodiilic vampire. I can't tell you how many topics this was posted in the forums, and I believe I might have been the first one to post it as soon as I saw this guy around Morthal

.
Long story short, Bethesda tried to fix lore by revamping vampires(no pun) and taking away their human-like appearance. They also took away stage 4 hostility and added some more lore to the native vampire clan.
QUOTE
I am having trouble with this one, and need some ideas. Why does your character drink blood? Why do they feed?
There are a number of reasons why.
1.Either they slip into a coma after a long time without feeding(don't even count game mechanics into this, they are irrelevant). QUOTE
"More than 50 years ago, my wife Rona and I were both turned into vampires. While I came to embrace the changes in myself, she did not. She hated what she had become, and refused to feed to keep herself healthy. She eventually slipped into a coma from which she has not awoken."-Janus Hassildor
2. They risk losing their sanity and survival.
QUOTE
"I rediscovered this diary today. It has been 13 years since I last wrote in it. With an eternity before, and the blood hunger ever pulsing in my veins, there is little urgency for diaries, or much of anything.
Has it really been 89 years since I last wrote? The pages are getting fragile. I have rediscovered purpose, though it took nearly a century. I have finally gained some measure of control over the blood frenzy. I think I will try to establish a life among the living in one of their great cities."-Jakben's journal.
and
QUOTE
"The worst effect, of course, had to be my blood lust. If I did not kill once a night and drink blood, my hunger would gnaw at me, and any wounds I suffered would not heal no matter how much I rested."-Vampires of Illiac Bay.
and
QUOTE
Entry 8: Two weeks. Two weeks have passed since Luktuv locked me in my quarters. Try as I might, I cannot free myself. I cannot breach the doors! If I don't feed soon, I feel I will go mad.
Entry 9: Food blood blood blood blood I need it I need blood need blood."-Lord Lovidicus journal.
3. Discretion.
QUOTE
"To preserve our ideals and way of life, two primary edicts shall be observed. Above all, reveal thyself and our Order to no other, for discretion is the greatest of our virtues. Do not feed where you may be found out, or on those who may not suspect your passing. Avoid daylight by lifestyle; dispel common belief in our kind, and maintain supple appearance through satisfaction of the thirst."
When they say "maintain supple appearance," they mean feed until you look identical to a mortal. To the point of being indistinguished from a normal person. No way to tell the difference. The quote was taken from
this ingame book which states the tenets of the Cyrodiilic vampires.
RP-wise, my characters Draken and Raven feed because they know how prejudice mortals can be and how fearful they are. They understand that in order to manipulate and control the masses they must assume the position of a mortal rather than a blood-sucking vampire.
Raven is more politically gifted. She feeds because she is always in the company of nobles, politicians and government officials and dukes and princes, etc. She was born a vampire and she grew up knowing nothing more than how to live her life as one. And feeding to her is just like eating to a person, except is has a much more crucial and meaningful value to her century-long career of infiltrating mortal society.
Why does she stop feeding? Well, in one case she had to get her hands dirty and instead of using a pawn or a thrall to carry out her bidding she had to take out a group of necromancers that were aware of her condition. And to make matters worse, they knew her personaly and were a threat to her secret identity as a vampire. She forgoes feeding, storms their location in the dead of night and well, does what she's good at. She knows she's at her most powerful there and in a cavern, where no witnesses would live to tell the tale, she shows her true colors.
She also feeds because she has a sharper mind when she does. She can negotiate. She won't be worrying about taking a chunk out of someone's throat while she's moving the pieces across the board game. She's part of the High Council and she's loved by the people. If she even showed her fangs once, she knows she would be hunted down relentessly for being what she is. A predator at the top of the food chain. It's as simple as that to her. In her mind, feeding is what keeps her "alive" per se.
Draken, on the other hand, feed because that's just his way of life. It's the whole "I was born a vampire. I will always be a vampire. I will feed as a vampire. It's my nature. Don't like it, don't care."
He's more of a military and influential type of fellow. Charismatic figure by day, dark Molag Bal virgin-sacrificing cultist by night. If he is blood-starved, he does it to go on a supreme power-trip against his foes in clandestine warfare far from society.
Feeding habits: They aren't as messy as those clowns in Castle Volkihar. The Cyrodiilic vampires saw how the Whet-Fang put their cattle comatose and harvest blood from them, and they even did the same with the Deepscorn Hollow prisoner in the Cattle Cell. Raven and Draken, being high ranking members of the Order Vampyrum in Cyrodiil, kidnapped beggars, plucked criminals from prisoners and jails, orphan children and placed them in underground blood-farms or cattle cells where they could easily travel through Cyrodiil and feed from their own source of blood without drawing too much attention. Why kill their prey when you can safely take blood from a donor who is forever sleeping peacefully? Better than going through the hassle of seducing, feeding and possibly dumping the body out in the woods.
And yet I am, and live—like vapours tossed.
I long for scenes where man hath never trod
A place where woman never smiled or wept
There to abide with my Creator, God,
And sleep as I in childhood sweetly slept,
Untroubling and untroubled where I lie
The grass below—above the vaulted sky.”