The monitor presentation of a platinum print is always especially difficult. The vignetting may be annoyed for you, because it immediately jumps into your eye. However it is no reason to reject it, for me it looks fine.
Yes...I would have preferred that the vignetting had been in the upper corners instead (and evenly balanced firth and left) -- just a personal preference if there has to be vignetting...from a historical perspective and all that. But the camera would have had to been raised a little to get the top of the falls unvignetted. But otherwise, a nicely seen image.