It's not allowed by Apple to use the vibration as you want to:
Vibration. It is not permitted to use continuous vibration in your apps – short bursts as warnings is all that is allowed. Don’t bother trying to set up a timer to keep the vibration going, it will cause your app to be rejected.
It's not the method, but the result, that's not permitted.
Source: http://10base-t.com/unofficial-appstore-rejection-criteria/