There are a lot of possible causes, but a common one is Lambda sensor failure (the sensor that measures emissions). A Vauxhall dealer should be able to plug a computer into the ECU and tell you the problem for no charge.
The most common cause of management light illumination on the vectra is either the Crank poistion sensor or camshaft position sensor,if you live near sussex bring it in and I will plug your car into my diagnostic kit for free