Bert, I wonder if the vignetting is a result of the angle that the light is reaching the outer part of the sensor. I think I read somewhere that digital sensors work best if the light reaches the sensor at (closest to) right angles and that film was more tolerant in that regard.
Just looked up the article, the problem is minimised in telephoto lenses (at the lens design stage) by lengthening the back focus and apparently lenses designed for DSLR's have longer back focus to allow light to strike the sensor at angles that are closer to perpendicular. If you're interested I could scan the article and PM to you.