VSS is needed for idle. The IAC resets position when the vehicle is rolling at steady speed on throttle. Without it no IAC position resets, the PCM loses position of the IAC over time and the startup and idle quality tanks.
I was starting to think that's what you meant, what with it being a '98 and all. Passlock is what that system is called. VATS is the old system with the "chip" (resistor) in the blade of the key; wasn't used in GMT400's.