[StumpedGMowner1]

This wont be vacuum leak related at all. So you can rule that out. Even a leaking intake gasket will throw lean codes not rich. Without viewing any live data off the pcm all i can do is give you ideas. Distributor cap and rotor worn, plug wires worn and leaking, bad fuel pressure regulator in spider leaking fuel into vacuum line, stuck open or partially clogged injector. O2S11 can cause this but in my experience its almost never the culprit. I would start with finishing the tune up with new wires and cap n rotor since you already started the tune up anyway and your getting an accompanying misfire code.