Aasmund, is the Fog of War in my project like the one you're going for?
I use Rojohound's Canvas plugin to save memory. Basically, set a "Darkness" layer that's black and non-transparent over the main layer. Put the canvas on that layer, then just generate "lightwindow" sprites (these are just black circles with the blend mode set to DestinationOut) where needed, then paste them to canvas and destroy.
I think I've explained that right (going from memory here!) but that's the gist of it.
EDIT - sorry, I think it's the canvas that needs to be set to Destinationout - the lightwindows can just be ordinary black circles (I use the paintbrush tool in C2 set to 0 hardness to get a nice gradient radiating out from the centre)