This device controls the lever on the TDI's variable nozzle. This is how it adjusts the amount of boost your TDI receives into the intake runner. If your TDI is feeling very sluggish sometimes it is nothing more than this control.
If it turns out your vacuum actuator is not your only issue, you may need a new turbo charger. Many TDIs are starting to have issues with carbon build-up in their EGR valve. If you are dealing with that problem also, we now have brand new EGR valves complete with the anti-shutter valve.