I hope you can do the questline as a vampire; my character has this whole "use their power against them" concept. She's a vampire that hates vampires and were-creatures and the undead. So it would make sense for her to be Dawnguard (especially since she's an archer, and we get crossbows now )

wat do u think the case maybe if ur a werewolf and would like to change to one a vampire??? do u think they will fight u if ur a werewolf???

that would be pretty awesome! Then I could play young youth who rarely shows emotion. He lives his life only to seek out and kill vampires that would harm humans. He is only known as a vampire hunter named D .

Why isn't anyone asking this question when they are interviewing these guys from Bethesda.

It's so simple: Can you join the vampires and play in your own style without having to go into this ugly vampire form, ever!?
If somebody captures a Bethesda employee, you shall ask this, thank you!

I believe this is called an oxymoron.

I suppose that the Dawnguard will display some wizadry and cure it. I mean its just a disease, and if your not choosing to use it for evil then i reckon they will help you.

Or perhaps, the cure will have to be taken in a side quest prior to joining the Dawngaurd?

you would think they would give you a killing spree with the vampire lord in the questline like they did with the werewolf

What I could see happening is;

Dragonborn: *Enters Dawnguard's stronghold*
Dawnguard: Stop right there! You blood svckers are getting courageous now aren't you?
Dragonborn: Dialogue Options;
1. No, wait, what are you talking about?
Dawnguard Response: " Oh, you aren't a part of their clan are you? Well what do you want? ( and then prompting to be able to join or just walk away )
2. Yes, I'm here to kill you all! ( Or something like that, leading to them being hostile )
3. What is this place? ( Leading to an explanation and then being allowed to join leading them to think you want to 'repent' )

Or I could be dead wrong. We'll see.

Seeing how there is little conquence, I don't see being a Vampire joining the Dawnguard a problem.