I just replaced the AC compressor and drier on my car, after the clutch bearing was making warning noises, and then the shaft splines stripped before the nut came loose.
The vacuum gauge kept indicating a relatively high leak any time the pump valve was turned off, but pressure testing it with nitrogen didn't show anything noteworthy, nor did testing with trace amounts of r-134a and the leak detector.
Apparently the gauge was flaky, and was acting overly sensitive, as switching to a different micron gauge didn't indicate as high a peak vacuum level, but also nearly no drop in vacuum when the pump was disconnected.
After charging the system with the recovered 134 and a bit to make up for losses, the vents blow cold air again.