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Some diesel engines (Toyota for one) have an alternator (belt driven as usual) with a vacuum pump attached to the back of it.
This provides a source of vacuum for brakes etc as a diesel engine provides none as already explained above.
This may help with your "vacuum-with-engine-off" idea.
Another option is to fit a vacuum reserve chamber as a short term vacuum supply which can be replenished after the engine is again running.
A normal car engine in good condition will provide vacuum equal to about 17 to 21 inches of mercury , normally expressed as in/Hg.
Apart from the uses already mentioned by Ollie a vac gauge can help identify and diagnose engine problems like mismatched carbs , timing not set correctly , worn rings and valves , air leaks and other items.
Google for what these readings look like with the gauge attached.
Pete.
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