And after a good long while, when you're comfortable with your unlife, if you can divine a reason for your character to go look up the Dawnguard in the first place(I couldn't), you can eventually join the Volkihar and engage in the cutthroat politics of vampiric royalty. Not before, though.
You could have him pose as a human to infiltrate the Dawnguard, either with the intent to bring them down, or to use them to help you find the mysterious Volkihar coven you've been unable to find by yourself.
Or, why not join the Dawnguard prior to becoming a vampire? I worked out some quick backstory for my new character this morning. He is a Redguard who's been long interested in dark magics, but since that's very frowned upon in Hammerfell, he's gone to Skyrim to investigate rumors of necromancers there. After learning a bit of conjuration and illusion magic, he hears about the vampire threat, and decides to join the Dawnguard, both to help out and to have some semblance of a home and a family in Skyrim. But he contracts Sanguinaire Vampiris in Dimhollow Crypt, and after fully turning, he realizes how much stronger his magic is as a vampire, and decides to remain one. This leads to him joining the Volkihar coven, again for a home and a family, and as a way to develop his skills even more.
