Interesting problem. It sounds like the issue is with the master vacuum switch in the door panel. That or the previous owner messed with the vacuum system. Others can chime in and offer their assistance here.
You can read up on how to replace it here.
http://dieselgiant.com/mercedesmastervacuumswitchinstall.htm
To disconnect vacuum for the door look system look inside the engine bay for a small two legged dashpot. It resides near the oil filter housing. At least this is the case on my 85 300D.
This post has a diagram as well as a picture of the two legged dashpot I was referring to.
W123 Vacuum InterLock Diagram
Fixing the vacuum door lock system isn't hard at all, assuming a PO didn't screw with stuff. I still want to believe that the problem is with the master vacuum switch in the driver's door panel however.