One of the most popular ways of preparing a vibraphone is placing tin foil on the bars. Tin foil creates a very nice buzzing effect, almost like a distortion pedal. This will only work with the pedal down.
Isolating individual pitches is tricky, as adjacent pitches may trigger the foil.
I prefer to put foil beneath the bars to prevent the mallet from striking the foil. This is less consequential when the instrument is bowed, or if the piece is slow enough that avoiding the foil is easy. On the other hand, it is preferable to have the foil on top if you want to place or remove it at any point during the piece.
The video below shows a few examples of what this effect sounds like.