I'm pretty sure no one has the answer. But still, what do you think will happen if you try to join the Dawnguard while you have the Sanguinare Vampiris disease?
I do hope it will be possible to be a vampire who still care for the human race and other races.
To be a vampire that does not follow his master's way.
Well, the Dawnguard may be rather inquisitorial about vampirism, so they may just end up lopping your head off and scattering your ashes at the crossroads if you try due to the corruption of your blood in their eyes.
In a role playing sense, joining the Dawnguard as a vampire can make sense because with the knowledge of what kind of evil that is out there,
and what that evil plans to do with the sun, you can't stand still and do nothing about it.
If you're role playing a member of the Order, you can justify it by rationalizing that joining the Dawnguard allows you to learn about a rival clan, and snuff them out. You can also be in a better position to take out your "comrades" after they've helped you destroy your other enemy as well.
And what if you have already joined the Dawnguard but on the road somewhere you get attacked by vampires, and you get infected with Sanguinare Vampiris disease.
What then?
Take a potion of cure disease, pray at a shrine, do anything you can to cure yourself before you die and are reborn as the monster you swore to fight.
Alternatively, turn and realize that you're still you and have just as much of a right to exist as anyone else. More so actually, as you have surpassed death. You're higher than mere mortals, so why shouldn't you view them as cattle?