If everything else that operates on vacuam still working properly? If so, then the problem is probably the line to to guage, and yes it is a vacuam guage.
If there is anything else vacuam operated, that isn't working correctly, you should be able to use this as a clue to find the leak. If the engine is running, it is obviously making at least some vacuam. Therefore, you should be able to isolate down to your problem. If you can't find it by looking around and tracing the lines, you may need to get a diagram to make any sense of it.
Since the vacuam is all the way to zero, you should be able to disconnect a few things and see where you have vacuam and where you don't. Just use your thumb over the end of a vacuam source line as your test instrument.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in