Let me explain this a little better. Your EGR valve is sticking open at times, which is fine when your driving, but when you come to a stop, it leans out the engine, and it stalls. It sounds like it's at the earliest stages of failure, so something as simple as manifold vacuum can cause the valve to not close all the way. That's why it restarts. The AC puts a load on the engine, so anything that might make it stall, will make it stall if the AC is on. You already tried unplugging the EGR, and you said it never stalled when the EGR was unplugged, but it did stall when the EGR was hooked up. The vacuum test you performed tells you that it's the valve that's sticking, rather than the solenoid that controls the vacuum to the valve. Time to replace the EGR valve, and make sure you clean the mounting surface, and use a new gasket.
Take care,
~Chris