I've been chasing down a vacuum leak in my 1982 300SD and have a couple of suggestions of a general nature.
1) Never underestimate the brittleness of nearly 20 year old rubber. I have taken to replacing ALL of the rubber junctions and a significant amount of the line itself. I've tested lines out and had them show a leak weeks later.
2) Start at the top. I had multiple leaks, in both climate control and the door locks. But, if you go up to the brake booster (I believe that's the right part), a golf tee can help figure it out. I pulled the line for the climate control off and plugged the line with a tee. The car stopped the next time I drove it (and every other time). It helped in tracking it down.
I still have a few more junctions (and a couple of door lock diaphragms) to replace, but at least the car shuts off for now...