Verify the Continuity of Vibrator terminals with the help of multi-meter, if it is faulty then replace the vibrator.
Clean the vibrator terminals and check.
If the above thing doesn’t solves the problem then check both on board connection terminals for continuity with the help of a multi-meter. Connect one end of multi-meter terminal with Vibrator terminal and the other with its respective capacitor. Check both terminals one by one, if:
The Connection is open put a jumper to restore the connection
If Capacitor is damaged, replace it.
If that doesn’t solve the problem then it must be Vibrator IC issue. In order to fix that, all you have to do is to:
Reheat or Replace Network IC, this is surely going to solve your iPhone 4S vibrator problem if everything else fails
