Vampires and werewolves go into a whole other category next to Witches.
Witches are real, they practice witch craft. To them it can be a religious practice, but others practice it out of curiousity, and so on.
Wicca and Paganism are both religions, but a Witch and a Wiccan or Pagan have their differences.
You say your a vampire/werewolf, but I can't say it's not true, because then I'd be assuming, which often gets the 'assumer' into trouble. If it's true, well, I hope it isn't as bad as people say. A cursed life of some sort. I hope you enjoy it, but I just have to go with my gut on these things.
I'm quite skeptical, but anything is possible if you believe- right?
To answer your question, I believe in Witches because I know some. My stepmother is an example of one, but I also know others.
Vampires, werewolves, and such? I'm not sure, but I hope I'll figure out my beliefs for them someday.