Use these APIs to set the iphone to vibrate

AudioServicesPlayAlertSound(kSystemSoundID_Vibrate);
AudioServicesPlaySystemSound(kSystemSoundID_Vibrate);

The first function plays a beep sound if the devices don’t support vibration. The second function on the other hand does nothing on unsupported devices.

if AVRecorder is running, iphone will not vibrate too.

Source: Stackoverflow