If you mean the white one with a black top on it and a torx type depression in the middle of it - it is the Power Steering fluid bottle. Mounted on top of the electric power steering unit
The oil filter canister on your Vectra 2.0 CDI is under the top cover of the engine. It is on the right hand side.
it is under the car,near the diesel tank
if it has air condition it is behind the pollen filter under the bonet
the fuel filter is located under the bonet on drivers side at back of engine
Bulk head passengers side. frontera b diesel filter is under the nearside front door
If it is a diesel then it is behind the radiator under the diesel filter, it is a awkward position!
Under the car passenger side by the rear wheel
below the air filter. under the front n/s wheel arch
Under the fuel filter housing
its on the oil cooler /filter block thats right under the alternator
The oil filter on a Holden Captiva Diesel can be located under the alternator. It is next to the engine oil sump plug and at the back of the car's engine.
Under the car, in front of the right rear wheel