Vibration sensor for arduino
I assume the OP wants the frequency of the vibration, which an accelerometer such as the ADXL345 won't give.

I have two workarounds: if the expected vibration frequency is low, say, under 100Hz, use instead an on/off vibration sensor (adafruit has them for under a dollar) and count the number of transitions. If it's more, how about a microphone and a band-pass filter?