I went to the Companions and became a werewolf. I went through the quest to kill the silverhand, and after that was over, and I had added 2 points to the werewolf tree, I discovered that I was still a vampire, and had just went into the second stage. So now I am both a vampire and a werewolf.
When I play and have already become a werewolf, I do not contract vampirism. But I have discovered that if you become a vampire, becoming a werewolf does not cure it like it is supposed to.
I have seen threads where people asked if you can be both. Well, I am both. Do not know if it can be repeated, but i do know that if you are a werewolf you cannot become a vampire unless you cure yourself. I also know that for me, if I become a vampire first, that becoming a werewolf does not cure it. At least for me it did not.
Just though I would share that with you.

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cure one or c) continue as a hybrid and defy both Daedric Lords, hereby subjecting you attacks from both princes.