How can you lighten the vagina lips?

The same way Michael Jackson converted to being white. By laser bleaching. This destroys the brown pigment in the skin and you'll have permanently pink lips.

It can be done temporarily by chemical means, but those chemicals are nasty and you have to keep doing it.

You are probably asking about inner lips, which tend to have dark edges, especially when long. You can have them surgically shortened if they are long so the darkest, outer edge is removed and the inner lips fit inside the outer lips. Its a simple procedure and should not require hospital time.

* Latin (WikiAnswers prefers us to use the formal names)
Labia Minora = Inner lips
Labia Majora = Outer lips