What if your characters already a werewolf? How will that play out if you want to join the vampires instead of the werewolves.

good question i was actually thinking the same thing i read that theres gonna be a skill tree or something for lycanthropy

theres a way to leave lycanthropy !!!but im afraid i spoilit all i can say is you need to end the companions. But if im a vampire whats the cure do be able to go with the Dawnguard??? that i just dont know!

To clear the Vampire disease :
Conventional method

• Ask any bartender about any rumors going around, and they direct the infected person to Falion in Morthal, who studies vampires. Traveling to Morthal while at Stage Four vampirism can be difficult, since hostile persons will attack the vampire on sight.
  
• Once there, he informs the vampire that a ritual can be performed to reverse the curse. A filled black soul gem is required first. The Black Star cannot be substituted for this gem. Falion offers to sell the vampire a Black Soul Gem, if they cannot acquire one through other means.
  
• Fill the Black Soul Gem by casting Soul Trap on a humanoid target.
  
• Meet Falion at the stone circle to cure the infection.
  
• Falion endlessly agrees to cure vampirism, so the quest can be completed multiple times if infected multiple times.