Your engine doesn't have a O2 Sensor.
(Caution BE SAFE)
If you can place your NB on Jack Stands high enough to slide under and observe the Turbo Actuator, you can watch it pull down solid against the Set-screw when the engine is first started. You have to be looking at it when the engine is started.
A properly functioning Actuator will move Up and Down in its entire range of motion.... Up with the engine off and Down with it idling. Of course, this is depending on what's going on inside the Turbo Variable Nozzle assembly. If it is corroded, loaded with soot, rust, etc., well, it could be stuck in position or have very limited movement.
The Vacuum pulls down the Actuator in Full Boost position at Idle. Pushing on the accelerator under load results in the N75 releasing Vacuum to provide the requested Boost. It operates opposite what you might think. The Spring in the Actuator is much faster than the Vacuum. So, releasing Vacuum gets the job done faster, by design. Letting up on the Throttle or as the ECU reduces boost demand results in the N75 releasing more vacuum until the Vanes inside the Turbo are basically shooting the Exhaust gases at almost a 90 degree angle to the Turbine blades which reduces its speed and obviously boost. Then, if you stomp on the accelerator, the N75 applies Vacuum to pull the Actuator down which moves the Vanes in a steeper angle to speed up the Turbine (increase Boost). Sometimes the lag in the Turbo boost is because of the Vacuum catching up with demand as well as the hot burning Exhaust gases doing their part.
Side note: The VNT Turbo does not have an internal by-pass. The angle of the Vanes is what determines boost which is controlled by the ECU via the N75, based on Accelerator demand.
Here is a good Video showing the disassembly of a VNT15 off an ALH TDI engine. The guy does a fairly good job explaining everything including doing logs VCDS to see what the demand is vs what the Turbo is actually doing.
