I recall that the vent flap is actuated by a cable, driven by a slide control
on the dash. I don't think it's vacuum connected. The cable supports often break,
allowing the outer part to move, hence giving no movement.
The vac hoses that I removed from my AU spec car were all a bit rotten,
surface cracks all over. This may be your issue. (Guy is talking about my old stuff)
The vac system is complex and extensive. Your car may have an engine-driven vac pump,
adding to the complexity.
Nigel