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.