I may be crazy saying this but once the car is shut off you shouldnt be able to hear a vacuum leak, there isnt any pressure in the intake manifold at that point, especially not enough to hear a vacuum leak. If you wanna test this theory, when you turn the car of tap the throttle for a second or 2, this will release any pressure left in the intake manifold.If you still hear the leak after that, then it is not a vacuum leak.