a friend of mine had this problem, the air came out from floor vents when under power. ie accercellating. He replaced a small one way valve located just behind motor, near top. small white and black one way valve. it cost about $15 and fixed the problem.
Yeah, its a vaccum problem. Check all the vaccum lines & connections for damage to the plumbing. Also might be an idea to check the vaccum ball thing, from memory its under the front passenger side of the bumper bar, near the windscreen washer bottle.
As brewster123 stated I replaced a small white and black valve behind the engine that is connected to a hose at each end. Worked a treat. Take it out and blow through both ends it should only go one way. It just pushes in and is easy to replace. $15 from holden spares.
Yes, had this problem before. Still got my VS Calais and when i accelerated the face vents turned off and the floor vents went on, take off the accelerator and you get your face vents back. Frustrating to say the least. I looked on this board before and did the vacuum line check, front ball check, check valve check LOL and finally someone told me to try the solenoid control pack below the radio. I was at my wits end by now so i changed it with a wreckers job. Pain in the ass to remove all the covers and facia but it damn well FIXED! it. Yeah. Check for vacuum leaks firsts as previous posts mention, solenoid control pack at the LAST resort (they not cheap) Cheers.