It's interesting that you mention vacuum. I've been hearing a lot of climate control actuator movement going on behind my dash. This is recent also beginning about the same time as the idle problem. It might be related. I mentioned this to the shop.
I'm going to have to talk to the tech who worked on the car and figure out how he came to the conclusion he did. If he didn't actually swap out the ECU which cleared the problem I'm skeptical. Or, if they swapped out the Idle Control Unit and this made no difference.
I don't understand how the Air Mass unit could be the problem though. Can you explain that? For example: the MAS unit says "There's more air coming through, the ECU responds by throwing more fuel through the injectors." This would result in an ultra rich fuel mixture because there is not more air coming through." Remember I've got a smooth running engine here under all loads and RPM's.