It always helps us to help you better if you specify the exact model. I will proceed under the assumption that it is a 123 car because you eluded to the vacuum pump shutting off the engine. On the 124 the locks are operated by an electric vacuum pump.

On the 123 it is a very simple process to troubleshoot. Start at the drivers door. Remove the panel and substitute the vacuum supply with the handheld vacuum pump. From the drivers door you will either go to the drivers side rear door or to the passenger side. When you get to the passenger side don't forget the gas tank door and trunk latch that are downstream from there. Just substitute the pump for the supply line until you find the leak.

The good news is that with just a little care, you can remove and replace the door panels on these cars with no damage.

Good luck,