You could have 1 of 2 problems, the first is very easy and inexpensive, it could be a faulty air flow mass meter which is a very common fault with the vectra. It is easy to replace and you can buy one on ebay for around £30.
The next is complicated and very expensive, which could be a faulty ECU and this will have to go to a vauxhall main agent as each ECU is coded to the car and will cost you in excess of £700 - £1000.
Either way I would suggest taking it to a local garage and have a diagnostic done on it first anyway.
Jason, April 2009