This from my Vectra manual. Depending on radio model, the display may vary slightly.
Switch on ignition.
Press and hold AS button and switch on radio. Display shows 'SAFE'. After 3 seconds it shows '10----'.
Enter the code number with preset buttons 1, 2, 3 and 4.
Assume radio code is 3650:
Press preset button 1 3 times (Display '10 3---')
Press preset button 2 6 times (Display '10 3 6--')
Press preset button 3 5 times (Display '10 3 6 5-)
Press preset button 4 10 times ('10 3 6 5 0')
Press AS button until a 'beep' is heard and the radio starts working.
If a wrong code is entered the display will show 'SAFE' and, after the waiting time, '9----' and so on.
DO NOT SWITCH OFF THE SET DURING THE WAITING TIME.
Waiting time is approx. 10 secs for the first two attempts, then 10 minutes and by the tenth attempt 640 minutes. After the tenth attempt the set is locked. They recommend having the set put into operation by a Vauxhall dealer after the 6th attempt.
I hope the code you have is correct!