Vibration is a form of touch. The difference between sound and vibration is frequency: sound sits above ~20 Hz in the audible range, vibration below it, in the tactile. Vibro Yoga works in the overlap zone, where something is both heard and felt.
Pacinian corpuscles in the skin are especially sensitive to 200–300 Hz, but the lower frequencies we use penetrate deeper — they are felt in the skeleton, not just the skin. That is why the vibration feels 'from within'.