Have you checked the rubber hoses going from the vacuum pump to the brake booster and also into the cabin where vacuum operates the internal vanes?
If these hoses have corroded with time and developed cracks or have come off then you could have a leakage of vacuum.
If there is no vacuum to the cabin then the air vents will only blow out the defroster vents...not to the side or foot vents.
Possible that the vacuum pump has failed or maybe brake booster is at fault if the internal aircon vanes are working properly.
Have you replaced brake fluid and bled the system?