I don't have all the answers (any answers

) Peter, but will be watching for people's thoughts with interest. From a dark sky site I find I can usually wing it better just using dynamic background correction in Pixinsight, but from a city location with greater skyglow or through my Ha filter the vignetting is too pronounced. I think I've seen Marc suggest a way of "backing down" flats to subdue the effect.
Sorry for lack of hard facts though