Genghis - velocity (inches per second) is the normal parameter for engine vibration. I'm no expert on vibration, but the velocity is the product of frequency (number of cycles per second) and amplitude (displacement). It is normally measured by an accelerometer mounted on or close to the engine case or cases where the main shaft bearings are located. It is aimed at monitoring the level of vibration generated by out of balance on the engine shafts.
As it is a function of both frequency and amplitude, it is a good indication of the level of "damage" being caused to the structure - low amplitude x high frequency can be equivalent to high amplitude x low frequency in terms of damage mechanisms like fatigue.
The latest engines have much more sophisticated analysis carried out either on board or in a ground station.