Here is a test I did with a vibration sensor quite a few years ago.
An offset weight was on a motor with the sensor attached like this:-
Then I changed the voltage going to the motor.
Attached is a small 3gp movie which shows when the motor is running there is no output until the motor reaches a specific speed when you can see the trace move. Ignore the gaps in the trace that is just the stroboscopic effect between the video and the oscilloscope trace.
I think this shows how useless a vibration sensor is for detecting an impact.
Vibration.3gp.zip (101 KB)