There is no vacuum connection to the Intake Manifold as your Diesel Engine has no Vacuum in the intake Manifold.
The line that starts at the very back of the Intake Manifold goes to a small overload valve on the Fire Wall and a line from there goes to the ALDA on top of the fuel Injection Pump.
One of the lines going through the Fire Wall on my 300D is Black and it connects to nothing because it is a vent line andis therefore open.
I believe the seat line is Blue.
Vacuum Diagrams look here if the site still works: W123 Service Manual
https://www.startekinfo.com/StarTek/outside/12265/?requestedDocId=12265
In the diagram below I believe the color represents the color of the strip that is on the vacuum line.
Except for the 3 and 4 way connectors the rest of the connectors are just short pieces of vacuum line that you could cut off a length off of yourself.
If you want something bettern then the typical vacuum hose you could use Silicon Vacuum Hose.
The Plastic Vacuum tubing can be replace by any nylon or plastic tubing that is aproximatly the same size as long as the connectors can seal on them.
But, replacing the plastic tubing means the loss of the color coding.