Could be lots of things on that car....sounds like you are getting pressure from the system, so doesn't sound like a vacuum problem - or you could not have enough pressure to move the actuator.

The black switch (the interior door lock) will always stay flipped out when unlocked or when the door is open (it will not lock when door open).
I would check for leaks and then lubricate the connections.

The other problem could be the vacuum reservoir - you are getting some pressure, but maybe not enough to make the door locks move. I think the vaccum reservoir on your car is under the hood (big plastic tank looking thing).