I hear the air release when the trunk is closed. Since the passenger door assist still works I will try to listen for it on that door.
I just relaized something else last night. When I get in the car and close the door to the point where the door assist used to work, the seat belt arm extends. So it thinks that the door is shut when it is not shut all of the way. Is the seat belt arm activated by the same micro switch in the door? If it is I bet you may be right and I have a vacuum leak or loose hose.