Chris,
Disregarding the fact that we believe your 1980 normally aspirated 300D to be a "Euro" model we would be using the diagram at: http://www.peterschmid.com/vacuum/1980/617_912.jpg... but IF in fact yours is a "Euro" model we need to use our diagrams carefully. The fact that all 3 of the vacuum connection on the single 3/2 switchover valve are in use, this is probably due to it being a "Euro" model.

Yes, I'm convinced that your VCV is on the outside of your "MRSF" type IP and it has two vacuum connections on the top with one having a restricted orifice below the " Y " connector. If like so many other MBZ components, I'll bet your VCV has a p/n and I'd love to have it. Judging from your photos and the shop manual I have, I'm guessing your VCV mechanically connects to your IP on the backside of the VCV. Sooner or later I'm thinking you will want to remove your VCV to clean it if for no other reason... and to see if its vacuum bleed-off action is still smooth over an increasing IP/throttle load. I'm sure the "Euro" models worked on the sam principal as the U.S. delivered cars, we just have to figure out how things should go from looking what diagrams we do have. I think your 1980 Euro is hooked up similarly to our later models but yours [thank goodness] does not have the extra ordeal of an EGR!

Keep finding and plugging those leaks... and sending those "pics",