y not beleive

Ydo ppl not beleive in vampires

Because they are a natural impossibility.

Yes. Impossible. Just like werewolves,growing wings, becoming a Mermaid; or changing somebody into a frog!

I would say impossible on a physical level.

Magick cannot do physical change, therefore you can't become a vampire. Physically most people don't believe in them because a. no one's ever met one and b. there's historic/scientific explanations to vampires. The only one most give a pass to is astral. [Vampires are from another plane, but they aren't physically real] keep in mind the astral isn't universally believed in, but if it's real, vampires would be like spirits/energy.

Personally I leave a small sliver of hope mythological creatures [Vampires, werewolves, meremaids] are real, but I'm 99% certain they are not. Either way, you are human, and because magick cannot contradict nature, you can never become a vampire.