to the OP - go for a drive, find a long hill somewhere, get to the top.
turn on your AC, aim down hill, bring the engine RPM up (3000-4000?) in 3-4-5th gear, and shut the throttle (engine braking).

this can make more vacuum than just idling and might force your HCV closed...

does the AC get cold?

Just for posterity and to follow up, there were 2 problems with the system:

1) The system was overcharged. I recovered the refrigerant, vacuumed it back down and recharged it.

2) I ran through all of the tests in the factory manual and determined that there was a broken wire to the interior temperature sensor. It would occasionally connect due to the normal bouncing of the car while driving. So, for a while I’d have cold air. But, when the connection broke, a flapper door would open, allowing hot air to pass through the heater core and through the vents into the cabin. At some points, I would be getting 100 degree air blowing out of the vents while the a/c was on full blast.

After correcting these 2 problems, I am now getting a consistent 36-40 degrees at the vents, and I will have my first comfortable summer in the 5 years that I’ve owned the car.

Thank you to everyone who contributed to the thread! I really appreciate your help.