Assuming you have no vacuum leaks....

What happens if you unplug the ais at a warm idle then take it for a drive? I had a suzuki with a bad ais, would run fine at cold idle but stall when warm. A replacement even online was around $200. I drilled and tapped the throttle return stop for a set screw just like a carb and drove for years without a problem. I would say try disconnecting the ais to find out for sure if that's what's keeping your rpm up. I don't know if chevy computers do it, however I do know the older mopar computers use spark scatter and fuel corrections to control engine speed, using the ais only if fuel/timing alterations is insufficient. I would first want to rule out the ais because it may be malfunctioning or something may be making the computer command for a high idle.

Your vss is probably the usual hall effect type. I don't know what the chevy standard is, many put out 8000 pulses(alternating from 0 to 5 volts) per mile. Some are 2000, 4000 or 16000. An ohm meter is of no help here.