I agree you're on the right track, it's just a matter of systematically finding the leak. The solid yellow line goes to the "master switch" in the drivers door that feeds the lock circuit (yellow/red stripe) and the unlock circuit (yellow/green stripe). If you put the mityvac on the solid yellow line (under the hood) does it leak down with the drivers door locked, unlocked, or both?

If the system only leaks down with the drivers door locked, you need to troubleshoot the lock circuit (yellow/red stripe) downstream of the drivers door.

If it only leaks down with the drivers door unlocked, the unlock circuit (yellow/green stripe) should be checked.

If it leaks down in either position, I would suspect the solid yellow line or the "master switch" in the drivers door.

Let us know, and we can give you some tips. I did this about 2 weeks ago and ended up finding a bad actuator. It's not very hard if you do it systematically.