Even if the evap purge was open to the air instead of hooked to the tank, it'* not going to make a big difference. Vacuum is going to fluctuate with idle a little. Being that you are likely using the AC ....and that dropping idle slightly and many people commonly asking that question this time of the year, I'm going to ask if you are running the AC. Cause..loading up the engine with the compressor is possibly going to do the vacuum drop and rpm drop...and it cycles.
As for the lean code. How do your battery cables look under the rubber boots? Connections at all ends of all cables nice and clean? I've seen old cables cause P0171 before.
00 Regal GS: All kinds of cool stuff.