The proper way to test the pump is to connect your MityVac to the vacuum port at the pump (use stretchy natural rubber tubing if necessary, the kind sold for tourniquets). Start the engine and see if the gauge on the MityVac reads down 25 - 30 inches and is steady.
I just replaced the Vac pump on my '85 300TD before it disintegrated, it was getting noisy, a clear sign the bearings were shot. Some prefer a rebuilt, I preferred to go new because the last one I tried to rebuild had a little bearing in it that was bad and wasn't available as part of the rebuild kit (it is the diaphragm only) so I wasted a lot of time trying to get a part that wasn't available.
And you're smart in not running the car until its replaced with a good one (presuming it is really bad) because they can and do cause engine failures.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!