First of all, the vibration is neither restrained, as it would be with a good tripod, nor damped as it would be held in the hands. Like the way a cell phone set to vibrate will jump around on a table but not in your hand.
An empty glass struck with a hard object will certainly vibrate, but you can't see it. Put water in it and strike it exactly the same way with the same force, and you will clearly see a strong disturbance in the water.
Your water test is useful as a comparative measurement but not much as an actual measurement.