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just thinking "out loud"
If it is not the vacuum pump (which is yet to be determined),
then I would suspect a leak in the line somewhere between where it enters the cabin and where it enters the instrument vacuum manifold.
This would explain why you only see the indication at altitude (assuming we are talking about a P-model).
The theory would be that pressurizecd cabin air is actually leaking into the system.
If this is true, you could fly it to altitude without pressurization turned on to see if you still get the indication. If this theory holds water, the problem would probably go away in unpressurized flight, since you don't get it on the ground.
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