There are a few tools you'll be acquiring as you own this car. The very first should be a Mityvac, or similar hand held vacuum pump. This will make tracing the vacuum system easier. The vacuum pump is on the left front of the motor. There are two fittings...a big one for the brakes, and a small one for everything else. The way you track problems down is to first check that you have good vacuum at the pump, then check every Y on the small branch with your vacuum gauge. You really don't need a diagram for this.

When I buy a new to me diesel, the first thing I do is replace every single vacuum hose and Y (not the hard plastic, just the rubber). They all will fail in time, so if one goes, you know the rest won't be far behind. A small investment of time and money will save you endless frustration.