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Yes Ric that is about the gist of it. In addition to vignetting, a flat frame will counteract the effect of dust on various optical surfaces within the image train. That is why it is best to take a series of flats for each filter combo and if you change reducers , barlows or powermates. Be awaye though as you move into the area of flat fields that if your illumination source is too axis critical, dust motes will look like 3d donuts instead of being completely removed. That is why sky flats or dome flats (sometimes)in partnership with a white tee shirt over the OTA end is to be preferred over a light box.
Having said all that is said, I'm not too certain that vignetting is a real issue with your image. It seems that there is a significant frame shift due to stacking and maybe a gradient issue. Try cropping the vacant area from the top, and crop the left to get rid of those two uranus coloured stars. I believe it will look much better.
HTH,
Doug
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