I am getting bad vignetting on the Mark III with a 70-200 f2.8 L II IS USM lens, it is however working perfect on my 50D, can anyone assist, I believe there is also a problem with the 200L lens
The 50D is a crop sensor so it's not actually using the light from the corners of the lens - which would hide any vignetting - on a full frame sensor like the 5D Mark III on the other hand it does use the entire lens surface, so some extra vignetting is to be expected.
When you say 'bad' I don't really know how to judge that - if you mean that you notice it more then on the 5Dmk3, then that's to be expected and not an issue at all (and the lens correction in lightroom will clean that up nicely), however if you mean bad then that could be a lens issue - for now my guess would be that it's just the first time you're seeing the vignetting on the lens though without expecting it, and there's really no issue.
I am getting bad vignetting on the Mark III with a 70-200 f2.8 L II IS USM lens, it is however working perfect on my 50D, can anyone assist, I believe there is also a problem with the 200L lens
The sensor IS larger on the 5D and vignetting is a very normal optical effect on any full frame body, you're just not used to using a large sensor or film cameras etc.
If you corp your picture to match the 50D you'll probably lose all the vignetting. Also what aperture and at what focal lenghts etc etc.
All my lenses on the APS-H 1D IV vignette, the 24-70, the 16-35 slightly and the 50 1.2 and 70-200 2.8 are the worst.