P1131 is indicative of a lean condition at Bank 1 (passenger side of engine). Could also be a bad sensor, but in my observations O2 sensors are just doing their job 99% of the time and reporting an out of range value to the pcm, as opposed to being a failing sensor.
I think you're on the right track with your suspicions so far (vacuum leak, etc), but something else I would check out is whether or not a leak exists at the midpipe to manifold flange. You mentioned that you are not running cats, so that leads me to believe that you're running an aftermarket midpipe (as opposed to stock pipe with cats removed)? If that is correct, did you use a new gasket at that connection? That gasket there should be replaced any time the midpipe is removed, so if the answer to that last question is "no" then I would strongly suspect a leak there.
2001 GT Convertible
8 cylinders, 5 gears, red.
6 cylinders, 3 gears, also red.
The devil hates a loser, and you thought you had it all.