Lubing the tumbler assembly might help, but if things are hard to move manually you might be fighting the vacuum or pressure that's in the system, which would make a vacuum leak less likely. Have you had a look at the electrical plug/connector on the pump itself?
Do you hear the vac pump run when the locks aren't working correctly? If the locks don't work right no matter where you operate them from then you can probably rule out a bad switch. If you have a bad electrical connection at the pump that might causing your problems. Or perhaps a bad pump.

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Fourings

84 Audi Coupe GT with Turbo engine, EFI (buh-bye CIS), Jamex seats, Koni's/H&R's, e-code lights. 210k and counting.
1992 Anthracite 400E, 161k, "Grey Ghost".
1986 VW Quantum Syncro, 192k, gone to the crusher in the sky.
1989 300TE 172k. (Was mine, now also Mom's car)
72 280SE 4.5 (Mom's car)