Just fixed my wifes 2002 1.8LS Vectra with this fault. Take the throttle body off, get some carb cleaner (or use petrol and a cloth) and clean the carbonised gunk from the body above and below the butterfly flap.
Also a common problem is the idle air by-pass port gets blocked up (like this one was). When you take the throttle body off, you'll have a small bore rubber hose going from the rocker cover to the lower right hand side of the body onto a small brass stub. This allows air to by-pass the butterfly when on idle and it gets blocked easily as condensation builds up in the hose and dries / hardens, causing the blockage. I used a small straight pick tool (you could use a thin, strong steel wire or 2~3mm drill bit) and cleared it out by working it through the crud in the stub. Give it a blow out with compressed air and spray some carb cleaner down it to make sure its clear.
Bolt it all back together, check the fault codes and clear them and roadtest. If it was the same as mine, all should be OK with no hunting and the emissions light should stay OFF.
All the best,
Graham.