Danomax, pic attached is very old log from my 1994 LT1 back in the days when it was stock and prior to any tuning. Ignore the DTC 77 as I was doing some home brew messing with the fan switching, prior to tuning the fans it. The log is at idle on a warmed up engine, in closed loop (BLM Enabled). Remember there are 19 BLM cells, idle uses cell 16, an will show any vac leaks as much higher due to the TB being closed and higher vacuum being pulled. Also when you plugged the PVC vac side of the system there is also a PVC input to the engine that feeds MAF measured fresh air to equal whats being sucked into the intake (keeps trims balanced). It's a hose on the right valve cover to the TB on the '94, not sure where it is on the '97. If you plug PVC vac to crankcase then any blow back will go thru that open hose to TB and could be why trims changed.
What are your readings at idle in BLM cell 16 ? . MAP , ST trims, BPW (injector Pulse widths), INJ DC (injector duty cycles), AFGS (air flow gms/sec), TPS%, TPS voltage, IAC position, RPM, IAT (input air temp) , spark advance and retard. I know your scan tool may not have all these diaplays but the more the better.
I'm Bob 1994 Z28, A4, cam, 1.6 RR's CAI, LT's, stall, cat back. Custom tuned. 2007 GMC Envoy, tuned. 2015 Kia Sorento, bone stock.