The BHW has the simplest vacuum system of all the TDIs sold in North America. That makes troubleshooting the VNT system easier. Less places for vacuum leaks.
The part that makes it NOT so easy is the location of most of the bits, specifically the N75 valve. The N75, the vacuum reservoir, and the tees and such are all mounted on a bracket down low in front of the engine, that is not easy to get to without a little digging.
But with a proper scan tool, you can easily check the VNT movement in Basic Settings with the engine running. It will cycle the N75 on/off in ~5 sec stages so you can just sit and watch the VNT actuator moving up and down.
If it doesn't, then you first have to check source vacuum. The source comes from the vacuum pump, same pump that provides boost for the brakes. So if the power brakes work, you at least know that part is good.
Locate the small vacuum line coming off of the booster vacuum tube back near the firewall. This goes across to the check valve (often called the yingyang valve, as it is black and white). Make sure that valve is working correctly. Should allow airflow one way, not the other.
From there, you'll need to remove the air cleaner assembly and the air inlet tube from the air cleaner to the turbo inlet, to gain good access to the rest of the vacuum system. You will have to unplug the crankcase breather heater on the breather tube and get that out of the way too.
Then you can look down towards the front of the engine, near the electric fan, and sort of see the rest of the vacuum system, and there is a hose coming back up and on to the VNT actuator.
The bracket that holds everything is held to the block with 2 Allen bolts. With a shallow socket, you can remove those bolts and lift the bracket and the sound deadening block over and out of the car, bringing the vacuum harness along with it.
The vacuum vent is the one that goes back up into the air cleaner lid. If there was a breech in this part, it would not cause a lack of boost, however it would over time allow unfiltered air to get into the vacuum system and clog up the N75, which WOULD cause problems.
Once you get the assembly loose, carefully check it for any obvious leaks, especially if the breather has been allowed to weep oil for a while. This can cause the molded vacuum fitting to swell and come loose.
Then with a vacuum pump, and a known-good chunk of vacuum hose, apply vacuum to the VNT actuator, it should pull up... and hold vacuum.
Of course, while you have this all loose, you can stick a finger inside the turbo inlet and see if the shaft is all flopping around loose. I doubt it is, but it is certainly possible.
Let us know what you find.