If you can hear the actuator closing and opening the air flap then it is not a vacuum problem. If there is no coolant on your passenger side floor your heater core is more than likely clogged.
I had the same problem: we drained the coolant unhooked the heater hoses from the water pump and flushed out the heater coil with an air compressor. Then flushed the whole system and refilled with coolant. Heat now works just like new.
Turns out that Dex Cool overtime coagulates in the heater core and can eventually clog it up.
1999 Z28, 3:23 A4, K&N Cold Air Intake, Hooker Longtubes, Custom 3" Exhaust, Flowmaster Cat-back, Injected Engineering Custom Tune, SLP Bilstein Shocks, Strano Springs, Umi Adjustable Panhard Bar, Umi Shock Tower Brace, 4 x OEWheels C5 Z06 17x9.5, Firehawk Indy 500 Wide Oval Tires, Meziere Electric Water Pump, 315rwhp and 328fpt