You need to buy a mini-vac, then find a vacuum diagram for your car and start testing.
The vacuum pump assuming that it works provides a vacuum to the brake booster then has feed lines usually from the main vacuum line to provide vaccum

To the IP shutoff level via the key switch
To the transmission to modulate shift pattern
To the locks.

The rubber connectors that connect all this piping together is prone to cracking over 30 years of life and heat, plus of course handling, disconnecting etc. Plus of course the rubber in the actuators which are in the IP, and each door, trunk, gas door.

If you've knocked say the line providing vacuum to the transmission loose this can cause enough leakage to prevent proper shutdown and lock actuating.

This assumes of course your vacuum pump which does wear out is actually producing enough vacuum.