Some vacuum gauges will have a positive pressure side as well. If that is the case, you could hook one onto the intake manifold because when you are in turbo range, the pressure will be in the whole intake manifold.
You don't necessarily need a vacuum gauge. If you are looking at boost, you could use just a straight pressure gauge. One that reads from 0 to (I guess) about double the max you expect to see.
I don't know if you would ever see a vacuum. In theory, no, because diesels don't have a throttle plate. But my friend's (1970s) mercedes (non-turbo) diesel does have a throttle
