i have had this problem in the past with a vauxhall calibra i once owned ( 2.0 16 valve ecotec engine ) the same engine that should be in your vectra.
anyway it turned out to be the tick over control valve. you can prove wether it is this by removeing the tick over control valve from the throttle body ( quite easy ) and soak it in a container full of parrafin over night, this should disolve any gunk and gum that has built up on the valve needle over the years. replace the tickover conrol unit and everything should be ok. if this works then i would think about replacing the tickover conrol unit with a new one in the near future.
In my teens I had a 1.4 SR Nova, a 1.4 SRI Corsa and 2.0 Vectra SRI 140... without fail, the idle control valve went on all of them, a couple of times on the Vectra.... My Vauxhall dealership cleaned them out for about �35 a time and ended up replacing it on Vectra free, even though it was a little way outside the warranty...