How do you push the front caliper piston back after changing brakes on vectra?

Use a "C" clamp to collapse the caliper piston.

Use one of the old pads across the face of the piston. If you use the clamp on the bottom of the piston it is possible to crack the piston. Using the old pad spreads the pressure to both sides of the piston.

That is if the piston is a cup. If it is solid on the face then it screws in, but I believe this type is only used on the rear brakes of cars.