The cruise control system in your vehicle is completely separate from any sources of engine vacuum and is controlled by a separate control unit. The entire system is completely isolated from any other and only uses inputs from other systems such as speed, etc.
All quite irrelevant since you have effectively removed the cruise control system from the scenario by completely disabling it.
Leaving very little to possibly suspect. There is no other mechanical control of throttle.
The components of the throttle body system are: the throttle body, a potentiometer that senses throttle position and movement, the idle stabilizer valve which meters air in order to keep a steady idle, an EGR valve for emmissions control, and a temperature sensor. None of the latter items mechanically manipulates the throttle body.
Without mechanical actuation of the throttle body and in order to achieve the power output you describe, the engine will require accurately metered air and fuel. I have no idea if the ISV or EGR, either together or separately, could introduce a volume of air equivalent to a fully opened throttle plate.
Nevertheless, air and fuel and accurate management of both must be present to achieve anything remotely resembling full engine power.
Do you recall noticing the position of the throttle pedal? Was it on the floor? What was the nature of your most recent throttle application?
The engine vacuum system on this model seems rather tortuous. There is a "damper", the function of which I have no idea, a solenoid valve of some sort, a "vacuum unit for intake manifold" which I believe is merely an tap into engine vacuum, and some check valves here and there. Then there's an entire subsystem built around a "suction pump" (p/n 078 133 753) which also taps into the intake manifold. By it's name it does not seem to be something that would introduce air into the system, but in your case, something is clearly malfunctioning so who's to say.
Finally, have you run the diagnostic output from the ECU to see if there are any error codes stored? This may give you some add'l insight.
I hope some of the above helps get closer to the cause. Give the age of the vehicle, I'm very doubtful Audi or the NHTSA will do anything whatsoever.
Ed