It sounds like your turbo is working. The variable geometry vanes of the turbo allow the turbine to respond more quickly at lower exhaust flows - like a smaller turbo, while maintaining the capability of a large turbo with a full exhaust flow. The vanes all move in unison but several vanes may stick and prevent the adjustment of the exhaust flow (or one if the fouling is significant). If stuck open, the turbine will respond like a typical large turbo - slowly with fuel economy losses from incomplete combustion of excess fuel dumped at demand into the combustion chamber (not enough airflow from the compressor induced with a low RPM turbine). If stuck closed, the turbine will respond like a typical small turbo - quickly but with high end fuel economy losses as the turbine is overboosted.
Here is a little video showing the actuation of the vanes through their range of motion.
The video is from the Turbo Reconditioning article with DTS:
Diesel Technician Society
The Diesel Technician Society website (Diesel Technician Society
) is very helpful for all of us.
In response to the question of whether some vanes activating while others not responding I would guess that a material failure would have to be present for this condition...a scoured vane (not stuck) would allow excessive exhaust flow or a damaged vane or unison ring damage could produce unbalanced exhaust flow across the turbine perhaps...certainly some vanes could stick and others remain unstuck but ordinarily the stuck vanes would prevent the unison ring from adjusting any of the vanes stuck or not.
Others may have a better way of describing this for you and you may use an OBDII tool to command VGT vane positioning as well.
Hope this was helpful!