A/C Vent air direction\Vacuum Problem

I have a 1992 Pontiac Grand Prix SE 3.1 V6. I had a problem with the vent controls. No matter what you selected on the vent control it would stay on defrost. I figured out that it was a Vacuum problem. I found a hose that was brittle and broken. Replaced it, and the a/c vents work now but when you accelerate, the vent control air flow goes back to the defrost. Even running at idle the vent air will not go only to the a/c vents. Instead it's like distributed/coming out all vents, just mostly the a/c vents. I know that I still have a vacuum problem but, I thought that the faster the engine went the "more" vacuum you should have. That's why this is so puzzling. To me it does not make sense. Oh, and the car does hesitate a little when you first start it up.(possible vacuum problem, right). Could the problem be at the engine itself, where the vacuum is produced? Thanks, Daniel

Actually the higher the rpms the less vacuum. Highest engine vacuum is made at idle. The vacuum source for HVAC comes off the intake manifold (upper).

On older model Grand Prixs there should be a vacuum cannister to help keep vacuum up or metered for the HVAC system. May look like a round plastic ball somewhere in the engine compartment.

Run an engine vaccum test with a typical automotive vacuum gauge (0-30 in hg). And check all vacuum line connections at the intake manifold.