I've heard that this is a common problem. Look at the post (and the others that say they have the same problem) to see what the common denominator is... The door lock tumbler itself!
Which door you you lock / unlock more than any other (in the course of a month / year / lifetime of the car)? THE DRIVER'S DOOR!
I'm told that the tumbler actually wears such that the actuator for the vacuum locking mechanism does not engage (release) when you open the door. My mom's 86 W203 (190-E 2.3) is also displaying this problem. I recently moved so haven't had the time or facility in which to tear the door apart to replace the lock tumbler.
I'll bet that you find this to be the problem, NOT a vacuum line.
Donny, any comment?