The central locking is run by a pump in the trunk, not engine vacuum.

Vacuum goes from the small line going to the brake booster, to the key switch, then to the shutoff valve. Sounds like you have a leak at the key switch.

The trim below the steering wheel is easily removed, pop off the plastic covers and you'll see the screws, and remove the gray plastic retainers from the bottom by giving them a 1/4 turn.

-J