I can back up what they're all saying about dark hair- I have really dark hair and when I was 20 I tried to bleach (with some hydrogen peroxide mixture my blond friend swore by) 2 really thin strips in it like Rinoa... it didn't work very well... actually it destroyed the hair I dyed and it never grew in dark again. I swear to god, luckily it was only like- 20 hairs, cause it grows in snow white now. Since I dye my hair red anyway I don't really care, but it means I can't let it go natural again til it's all white ^_^
Also, a few years later, I used some blonding kit specifically for dark haired people. I had to use 2, and never got my hair lighter than pumpkin orange. It actually looked kind of cool, but that was the end of my blond experiments!
But you must have blond hair if you planned to do Vash and hadn't thought about changing your color? I'd say you should really think about how long you want white hair... I'd go wig or paint over bleaching though.
If you still wanna bleach, here's some how tos: