A couple of things you can try:(posted 8349 days ago)If the condensors are adjustable, try settgin them up for the next larger neg size and retest. Or, use the 6x9 condensor to make a print from a 35mm neg.
Try printing a 35mm neg with the 80mm lens and the larger condensor set.
It is unclear, to me, is the vignetting on both sides, or only one side. You may have a mis-alinged lens, or carrier.
1/3 stop difference would actually be pretty good. This is may many printers recommend "edge burning" when printing.
Don't trust any tests without a negatvie in place. You may be seeing a reflection of the rear element of your lens in the lower condensor. This can have a serious affect on the apparent evenness of the light. Instead make a print with a processed, but unexposed negative in the carrier.
Report you results back here.