The key to achieving the optimal result in a vibratory tumbler is to make sure that the vibe is properly filled. Processing parts in vibratory tumblers is most efficient when the media and parts are able to tumble in addition to vibrate. In a tub-style vibe, this cascading action happens from one side of the tub to the other.
The video below demonstrates how a low media level simply vibrates the load and any tumbling that does occur is from the middle out to the sides. As media is added, it is clear to see how the media load begins to tumble from the right to left.