I've been following this thread for sometime now. Overall some really great discussion. Figured it was about time I chimed in with my thoughts.
I like totally agree with people that the volkihar vampires aren't volkihar. I also would propose that the character you play isn't dragon born. I mean so what if the entire plot, design, and dev disprove these (censored) theories, and it was created by people who over think simple things like a game because they are contrarians who feel superior when they go against the norm.
Insulting people rather than countering their claims with evidence is fun isn't it? Straw men arguments are also great.
For the record, I am among those who do not believe Harkon and his clan are actually Volkihar. Here are the things that led me to that conclusion:
1) Harkon and his clan never refer to themselves as such.
2) Neither does anyone else.
3) Serana mentions they are not the first residents of the castle, implying it could have been the home of the original Volkihar line who at some point were run out of the castle.
4) Harkon and his clan don't exhibit any of the powers recorded in "Immortal Blood,"* nor are they in the correct location. Castle Volkihar is in the western half of Skyrim, while "Immortal Blood" states that the Volkihar are situated in the eastern part of the province, to say nothing of the fact that they live in a big old castle instead of beneath frozen lakes.
5) No new texts have emerged to indicate that "Immortal Blood" is inaccurate/fiction/non-canon, etc. No new dialogue that I've been made aware of questions its validity either.
6) Harkon's claim of his clan being "among the oldest and most powerful vampires in Skyrim" does not necessarily conflict with "Immortal Blood." The book clearly states that the Volkihar are the most powerful clan in the east. Harkon's clan could be the most powerful clan in the west.
Those are the pieces of evidence I use to support my stance that the vampires encountered in Skyrim are not Volkihar, but a separate strain entirely. As someone pointed out earlier, no side can be proven correct until the devs decide to release a statement clarifying this issue. In the meantime, I will continue to use these pieces of evidence as the basis of my beliefs, either until Bethesda says I'm wrong, or someone provides convincing evidence to make me think otherwise. I acknowledge that I don't have a stranglehold on the lore of this particular field (or of the lore in general for that matter), and there may be very valid arguments against what I'm using as evidence.
Just the fact alone that Harkon's residence is called Castle Volikhar implies and leads me to believe that Harkon and his court are the original Volikhar Vampires.
And to (briefly) counter this:
Some look at the fact that Harkon and his clan live in Castle Volkihar as evidence enough of their bloodline, but I don't think that's the case at all. After all, Ulfric Stormcloak lives in the Palace of Kings, but that does not make him High-King by default (and he
never will be in my game), and he never renames it Stormcloak Palace or anything like that either. There's just as much precedent for people keeping the name of a castle/palace the same as there is of changing it. The name of a building doesn't necessarily reflect the residents inside. After all, Buckingham Palace isn't called Windsor Palace today, and it wasn't named Hanover Castle or anything like that when Queen Victoria moved into it in the 1830's. Not saying it's an impossibility, but it doesn't convince me either.
*I'm well aware that gameplay does not equal lore, and that this is not the strongest thing to stand on especially considering how screwed up vampirism became in Skyrim/Dawnguard. Vampires that should have been members of the Cyrodiilic strain are identical to those that are presumably Volkihar, which are identical to Harkon sans transformations, etc. It's a jumbled confused mess. However, the fact that they don't exhibit any ice-based abilities whatsoever other than a resistance makes me look at it as a point against the argument. After all, the complete lack of these abilities is much different than other gameplay does not equal lore points, such as the small population of Skyrim or the cooldowns with the Thu'um.