That's one heck of an image there Tj ... plenty of detail. Perhaps you should try using flats to deal with the vignetting. I'm not up to parr on the procedure but others in IIS are ( I'm sure someone will tell you about it ) and there are articles on the subject in the projects and articles link top left of site menu. The only other thing you could try would be GradientXTerminator but with such a full frame I don't think it's the answer ... it really works best when you have some space around the subject.
Good work !
p.s. I just did a quick GradientXTerminator filter on your big image and reduced it etc to post. You can see what effect it has had. I still tend to think flats may be a better solution/more subtle but that's up to you and others opinions.
