Hi DerekJust thought I would clarify what,s already been said and most of the advice is correct.
There is some confusion on engine types
VECTRA B:
1996 to 1998 model years Have X18XE fitted and use Simtec 56.5
1999 to 2000 model years have X18XE1 fitted and use simtec 70
2001 model years have Z18XE fitted and use simtec 71.
All suggestion about cleaning ISCV and modified breathers relates to X18XE / smitec 56.5
X18XE1 do not have a ISCV , they use a Throttle valve actuator with combined TPS. The Idle is regulated with direct servo control of the throttle butterfly,via pluses width modulation.
Therefore you cannot adjust the the TPS volts.
So is your engine a X18XE or X18XE1
The X18XE1 / Simtec 70 ECU's are not very good and inlet air temp circuit failure is common and as Tony as correctly suggested will fix this fault.
But they are also know to suffer from stalling faults witch normally is a combination of dirty throttle housing/butterfly , faulty Throttle bodies or ECU software update needed.
The Later Z18XE have no throttle cable and are a complete drive by-wire system. These also have problems of going into limp home mode or stalling and MIL on. DTC normally flagged are P1550 or P1555. Normal fix is to clean out throttle body and reprogrammed basic settings but sometimes the throttle body will need changing.
If you ever need any programming or coding on vauxhalls give me a call i have full dealer level. This also goes for MG/Rover now CD Brammals have gone.
HTH
Daz................