The carbon buildup makes the vanes stickier, and although they can move easily when not loaded (at idle) under part boost they stick, the vacuum actuator can't put enough force on to move them throught the soot.
What you need to do is get rid of the soot.
One way to do this is by getting the turbo hot and keeping it hot for a few minutes. You're trying to burn the soot off, so part throttle is needed - at full chat there is no oxygen in the exhaust gas and you're on the threshold of producing more soot.
So you want to work the engine hard at medium revs. Climbing a long slope is the best way, if you have one (A-road or motorway) near you. Otherwise - well, maybe get four heavy buddies in the car and go for a 20-minute run down the motorway at the speed limit.
The two major soot generators in diesel cars are full-throttle motoring (specially if remapped) and the EGR system. Full chat is limited by the smoke map on the ecu, as adding more and more fuel will result in smaller and smaller power gains but massive increases in part-burned fuel = soot. Diesels are rev-limited by the combustion process, the fuel simply can't burn fast enough, and any attempt to up the revs results in soot.
The EGR system is there to reduce NOx formation at part-throttle high-load conditions (mostly). These are the conditions that generate maximum combustion temperatures, hot enough to burn the nitrogen in the inlet charge into nitrogen oxides, NOx. EGR reduces combustion temperatures by diluting the inlet air with an inert gas - exhaust gas. It is a foul concept, brought about (IMHO) by the requirements of the Californian market driven by left-coast greenies. I don't subscribe, the methodology is flawed.
BUT (and it's a big but) EGR increases soot formation. The pollution legislation recognises this, calling for a balance between NOx reduction and particulate increase. The other main drawback of EGR is intake fouling, which is the reason why my EGR is switched off.
