Why do doctors say its bad for women to get there vagina piercing?

Because...: "we" say this because of the severity of the complications when it goes wrong; and the potential risks of severe infection.

Clitoris or labia: You probably mean piercing of clitoris or labia. Vagina enters the body and i doubt anyone but a pervert or wierdo would pierce or have their vagina pierced. Clitoral or labial piercings are unhygienic, good enough reason to discourage.