So then I assume you have Adobe Bridge installed, this may or may not work but you may want to try it. When you open the photo in Bridge, go to lens correction, at lens vignetting amount toward the (light vignetting)positive numbers and then widening the midpoint. I has worked for me on a few photos were I wanted the outside lightened without changing the centre area. It may not work but it may be worth a shot. Play with the amount and the midpoint.