Don't know if all Vauxhalls have only cold air from the middle vents, but my MK2 Astra does... And I couldn't turn it off completely! Especially in the winter this annoyed me terribly!! And besides that it made a nasty whistling sound when I turned the knob to the closed position....

So I went to search for the cause and found some pulverised foam which used to be the sealing edges of the valves. I still had some packaging foam and cut some of it to fit around the valves, problem solved and didn't cost a penny!!