How much vacuum at idle?

The only thing that i can think of is your original idea of a vacuum leak. One thing you can do is get a small bottle of propane and hook a hose up to the valve and poke the hose around different parts of your engine. If nothing happens, then you don't have a leak, but if the car suddenly runs better then you know you have a leak in that area. It may just be the F-cam with your combo doesn't like idling or low rpm's at all like jimberg said. Good luck!