i never had a scanner to verify but i had the cat delete option on my 89 c1500 for many years and by the 20+ mpg i was getting i doubt there was an issue. never got a CEL anyway.

i think the vacuum leak sounds logical. i dont know where your maf is but any leak after the maf would cause issues. so if its in the air tube it can expand after a while and leak. or if its in the tb like most of the 3800'* any of the sensors or tb base gasket might be bad. try warming it up until back in open loop, and spraying carb cleaner around suspect areas and listen to the engine

I don't think a vacuum leak would put the car into open loop. I think you would actually see your LT and ST fuel trims go rich as the ECU tried to compensate for the perceived lean condition the O2 sensor would detect.

But then again, I am not 100% sure. Certainly does not hurt to go over all the vacuum hoses to see if yoy have a leak.

If the vacuum leak causes a misfire then there will be unburned oxygen and hydrocarbons passing by the oxygen sensor so couldn't the unburned oxygen create havoc with the sensor?

A vacuum lead will not typically cause a misfire. Also, if you had a misfire it would be throwing codes.

you check/clean the o2 sensor connector and the pcm where it connects to

This is 1987 OBD1, don't think is has a code for misfire does it??
Regarding the connections, yes - I did a continuity check on the leads including the O2 sensor ground from the ECM to the engine block and all is fine.
The signal from the O2 sensor starts off fine and consistently goes bad after 10 minutes so what else could be causing that repetitive pattern? I don't know what else to check other than a leakage of air through the engine to the sensor - getting pretty confused about this by now, even more so than normal What about that exhaust pipe leak I mentioned - I'm going to be fixing that right away anyway so guess that will answer this question.

The leak is downstream from the cat and is not a factor. Isn't code 48 a misfire code?

There is no code 48 for this ECM - doesn't show on the scan tool and I also have the GM Buick manuals for this year which don't reference a code 48. Guess I'm working with a port fuel injected dinosaur here . I should also mention that I have also changed the plugs and ignition wires within the last 2 years so I don't think that'* the source of the problem. My scope is capable of monitoring the secondary firing pattern so maybe it'* worthwhile to have a look at this during the 10 minute period in question to see if any misfires correlate with the O2 sensor hash?

I would focus on looking for vacuum leaks as a likely suspect. There is also the possibility your car is acting exactly as it should, although lack of fuel trims still has me puzzled.

Sorry, forgot to give you an update on the trim observations.
There are in fact some trim adjustments occurring but only for the short 10 minute closed loop period. Apparently the INT functions only when in closed loop and the BLM monitors the INT for a longer term perspective so it does have some permanent adjustments stored in memory. My scanner is difficult to use for visually monitoring these values without extreme distracted driving issues and I also can't see all of the block adjustments with the scanner so I've just setup a laptop with data logging capability so I can take a test drive and look at everything after-the-fact. I'll let you know what I find using this approach as it is much more thorough. I should be able to extract all of the MAF/RPM BLM values for a clearer idea of what'* happening.