Hi Alex. Vignetting is a curious thing which is very rarely resolved totally with increasing the focuser diameter. One of the main causes is the fact that the light is focused to a spot on the CCD and as such there is nearly always an illumination gradient or vignetting to the corners of the larger CCD's. The only real fix is flat framed images to correct the gradient. My Fs 102 has a 2.7" focuser but still suffers some vignetting which is easy to fix with Flat frames.
I have found an interesting option to use for flats and a light box and will be investigating it further. I have some reports that it works terrific. Time will tell.
http://www.elec2go.com.au/webcontent8.htm