Were Cyrodiil's vampires described as being like they are, anywhere before Oblivion? If that deal with Clavicus Vile came about with the game, the lore would be just as flexible as the gameplay (until the game was actually finished).
Gameplay wise, they had the mechanics backwards, like I mentioned before. You become a vampire, but don't get to really enjoy the power of being a vampire unless you stop acting like one (ie, stop feeding on people). When you do want to play like a vampire, then you lose all the power of one.
http://www.uesp.net/wiki/Oblivion:Manifesto_Cyrodiil_Vampyrum
"On Our Dual Patrons:
To Kin-father Molag Bal, who brought forth the Bloodmatron Lamae to spite Arkay, we owe our existence, as do all vampires, though not all honor Him. For him we revel in the feast, and acknowledge the gift adrift in our veins. To patron Clavicus Vile, beacon oer our affairs, we owe our successes and social stature.
Our bond with Vile makes us unique among our kind, for his guidance steels our savage craving with reason and savvy. For him we live amidst mankind, and twist them to our will from offices of power."So, the bond with the Daedric Prince steels their savage craving with reason and savvy. In more simple words, they feed without the "Orgiastic" abandon like crazy blood-addicted vampires. And the "For him we live amidst mankind, and twist them to our will from offices of power" is a good reason to believe they live to manipulate people and they can only do so by feeding.
"And maintain supple appearance through satisfaction of the thirst."-If they want to walk among the living, they have to blend in, only then can they not be hunted down and shunned by humans. But what if the vampire wanted to be stronger, faster and more powerful than normal? He would have to undergoe painful days of starvation from blood and otherwise risk exposing himself/herself to society and even burning in the light of the sun, but nevertheless, they are more powerful, at a price. This is where we can speculate that Clavicus Vile did indeed alter their bloodline, but the price for being able to masquerade themselves and even walk during the day is that they must suffer if they want more power.
But if I see you're point about the gameplay! I somewhat understand. The backwards gameplay is a bit confusing. Like you said, they must be something different than they really are to be more powerful...something entirely different, but they know what they are. Kinda like the Story of the Masque of Clavicus Vile, where the woman had his mask to cover her uglyness, she knew she was horrid in the eyes of many, but she lived as if she was attractive, she adopted a new life until Clavicus Vile claimed back his masque.