Really, besides the few vampire enemies named such there is nothing to suggest the vampires you meet in Skyrim are Volkihar. They don't have any of their described abilities, and the abilities they do have are more akin to The Order. And the only book that does mention the Volkihar is careful to point out they only had (200 year ago by this point) a strong presence in only part of the province, not all of it.
Movarth is of Cyrodiilic bloodline, that is a given. It was a developer oversight to add him in. Now, Sybille? I would like to believe she is a Cyrodiilic vampire, until Bethesda added glowing eyes to her. Then again, the vampires of Oblivion had a distinct look about them.
Truly, if these "Volkihar" where the Order, they would not be all hanging out in castles. Perhaps they would congregate there, but most of them would be dealing with the crumbling Empire, perhaps trying to find a way to destroy the Thalmor to facilitate Imperial control, which they took advantage at the start of the Third Era.
I'm not saying they are the Volkihar, but I don't believe they are The Order, or the clan itself. Harkon says they lived in the castle for centuries, far from the cares of the world. The Order are directly involved in worldly affairs; politics, nobility, aristocracy, etc. This was evident in Oblivion where the named NPC's all walked around towns masquerading as humans. And all of them had influence.
The "Volkihar" are the boogeymen who kidnap people and enthrall them, lock them up in cattle cells, and feed on them on tables. They have their own political court, and so share some similarities to The Order such as hatred of thin-blooded vampires, or pinning the blame on others using clever deception(as the player needs to put a note on the body of a victim under the guise of the Dawnguard, as Seridur blamed Roland for the murder of Relfina).
But what remains similar? The power of seduction. The power to become invisible. But gameplay-wise, the Cyrodiilic vampires have many more bonuses, but since we're talking about a lore standpoint, this means little. Of course, when Harkon says that they lived in the castle for centuries and far from the cares of the world, it could be the time that it took from Oblivion to Skyrim; two centuries ever since the Empire collapsed, which would make the Cyrodiilic vampires want to rethink their plans. It is implied they aren't native to Cyrodiil, but only took over when Tiber Septim unified Tamriel. Knowing Cyrodiil is a seat of an Empire, they simple took over it to gain access to whatever they wanted; coin for their coffers, better lifestyle, "diplomatic immunity" as in the case of Janus and the Mages Guild. By all means, they are the free-loading wealthy power-hungry nobility of the Empire, who's arrogance is increased by their immortality and vampirism.
At this point, I wish a developer who made the vampires in Skyrim would clarify.