You don't say what engine size you have in your Vectra but from what I do know if you check the site below it will help you to check if you need a different car
http://www.cuddles.abelgratis.net/kerbweights.htm
you need to find the maximum weight of your caravan (on the plate by the door)
then as a guide find what 85% of your cars weight is with full tank and driver (in the handbook)
That is the approved towing weight of the car and the van shouldn't weigh more than this for safety
It's not the ability to pull a van that's the main problem it's actually STOPPING the van safely that is the hazard