It might be as simple as a vacuum leak in one of the lines.

If everything operates normally with the drivers door locked, then you can rule out the door locking system.

You should then check the lines under the hood and make sure they all hold vacuum. If not that is your issue and you can repair the line or the connector that led to the leak.

Vacuum system leaks just require time and structure, basically a process of ruling things out, and isolating the problem.

Do a little diagnosis with a vacuum tester and post the results.

JCD