buchacho:
It's a good simple question. Re-read it.
No, you reread my answer 
As I said, my answer is as good as the question. If you don't like the answer it's time to expand the question Or at least give more information. Why does the Arduino needs to be off? Only the Arduino? Is the power consumption a problem? Do you need a certain level of vibration? Does it need to stay on after vibration? And I can continue...
I can think of a hundred ways to turn on an Arduino with vibration. But they would not all make sens in all situations.
But if you're thinking about saving battery, just forget the whole turning off the Arduino part. Any external circuit to do so will take more energy then putting the Arduino to deep sleep and let it waking up to "vibration" for example with a vibration sensor.