now since dawnguard was annouced i have yet to hear anything about if you have previously cured your vampirism in the original content would you be allowed a second chance to become a vampire if you got the dlc or would you be force to become dawnguard???

you can repeatidly contract vampirism and cure it already. as far as i know.....

Would be pointless. I'm sure they'll allow you to turn vampire, since that's a pretty important choice that you can make in the game.
And as the other guy said, you're already able to contract and cure Vampirism repeatedly, so no worries there either.

Yes, you can become a vampire as often as you wish. That is what I've got from it. I also heard that if you are a werewolf, you can go to vampirism and then back to lycanthropy afterwards.

They did Dawnguard right, if this is true.