There are a few vibrato techniques I know of. One is to actually move the string up and down very slightly. Like a hardly noticeable bend. Another technique (the one I find a lot easier to do) is to slide your finger to the front and back of the fret which you want vibrato on. You have to do it pretty quick, and you have to be sure not to slide to a different fret. The vibrato I use is sort of a mixture of both. I just release all fingers (except the one in the fret I want vibrato on) from the fretboard, and shake my hand around like a maniac while keeping that one finger firmly on the string.