When I learn how to post pics, I'll stick some up of several VNTs that I have worked on.
Typically, the problem is not with the VANES (actually, I've never seen a problem with the Vanes). Directional Vanes, since they change the angle of exhaust gas hitting the Turbine blades of the Turbo. The issue is that exhaust gasses pass around the 9 levers and get inside the rotating ring that operates the Vanes........ you will see this in the pics. The crud builds up and eventually causes it to become "sticky."
You can go to TDIclub.com and search my Albums for the VNT 15 photos. I think this link will get you to the VNT 15 photo album....
TDI VNT 15 - TDIClub Gallery