If the engine management light has been on the ECU will have stored the fault codes.
Has the garage read these codes, if so what are they?
If not, get the codes read and tell us what they are.
I'd be taking the car to a different garage, they sound like a bunch of clowns, swapping parts at random and ripping you off because they haven't a clue.
How much have you spent already on NOT getting your car fixed?
It might well be the airflow meter, but have they tested it using diagnostic "live data" or a 'scope?
Have you tried fitting one off a vectra which is running well?
Try unplugging the AFM and take the car for a drive, if the AFM is faulty the car will run better with it unplugged as it goes to default settings..
What year is the car, you might be able to read the codes yourself.
i hate cowboy garages, August 2009
