I had a pimple and i put white vinegar on it. it went away but it left behind a white puffiness. now a few days later it has turned into brown flat skin. what can i put on this to go away? how long will it take to go away?

Depends on skin type: You have caused a mild acid burn or "frosting", thus the white puffiness by using vinegar (acetic acid). The end result is a dark spot (hyperpigmentation). How soon it fades will depend on your skin type (i-vi), sun exposure and how dark the spot is. Often takes a few months. Otc bleaching creams may be of some value.