Here's the exact manual reset procedure taken from the Vauxhall Dealership manual for the Vectra C (bold type below). Worked fine for me but you need to follow to letter. Most important to start procedure with ignition off.
Resetting the flexible service interval (ECOService Flex) with TECH 2
The display for the flexible service interval (time and kilometres) can be reset via the instrument assembly. The engine control unit communicates with the instrument assembly.
Ignition OFF. The tachometer display must indicate the trip odometer reading.The time counter in the instrument assembly is set to 24 months. The engine control unit receives a signal via the CAN bus and the "oil condition" is set to 100 %. The engine control unit and the instrument assembly are now reset.
Press and hold down reset button (trip odometer button)
Switch ignition ON and wait until the service interval display changes and keep the reset button held down for at least 3 seconds.
After 3 seconds, the tachometer display indicates the symbol with the maximum mileage, 34.000 km for petrol engine or 50.000 km for diesel engine.
Notify the customer that a service display accidentally reset by the customer can not be updated again by the Opel dealer. Depending on the mileage driven since the last engine oil change / Main Service, a new engine oil change may be necessary in order to prevent engine damage