p reg vectra used car fob but it wont pair?

The key neeeds to be "coded" to the car using diagnostic equipment such as tech2, vaux-com, op-com etc.
Phone around for best price, independent garages/diagnostics specialists are usually cheapest.
You will also need the "car pass" card, which is about the size of a credit card and has the key, radio and immobiliser codes printed on it. If you dont have this you'll have to pay a Vauxhall dealer about £25 for a new one.
The ignition on/push the button re-sync only works with keys which are coded to, and recognised by, the security system, otherwise anyone could drive off with your car!!

vaux
August 2009