Pretty sure from what I heard you have to start by joining the Dawnguard, and then after a few quests you get the option to switch sides.
This raises the question though... Anyone who actually has the DLC and knows- Can you join the Dawnguard if you're already a vampire? Or do you have to get cured, join the dawnguard, and then join with the vampires and end up re-vamped?
i want to black out the sun!!!!