I'm trying to picture your scenario and just can't seem to get it to "work." If a lens' view covers the total negative, then the corners won't be vignetted as badly as the OP's as far as I can see, no matter what the magnification. That is, unless there is an illumination issue, or something is blocking the light path.
Yes, I agree with you, that there is a certain amount of falloff with most enlargers (the need for edge burning as SOP confirms this in most cases), and I agree that, if possible, the area of the negative that one wants to print (the cropped portion) should be as nearly centered in the lens' field of view as possible. But, even with falloff, the corners should not be totally white in the print (i.e., almost no light at all getting to the paper) unless there is a lens coverage issue.
IIRC, the OP stated that he/she cropped out the right hand corners, which also were vignetted. If that is the case, then indeed all four corners are vignetted, and that 75mm lens just won't cover 6x7. Or, he/she has one an illumination problem like I mentioned above.
I print with both Omega DIIs and Beseler 45ms, but with diffusion sources. With a lens that is adequate for the negative size I am printing, I just can't seem to imagine any way the corners of the negative would get vignetted as long as the light source covered the negative adequately... I can move the easel so that half the print is "in the dark" and the negative image still shows a the clean edge of the negative carrier. Corners are fine, even at high magnification. I can place all four corners in the middle of the print and they are clean and relatively well lit. Maybe it is a condenser issue/lens placement issue as Bill suggests, but once that is fixed, then there really should be no problems getting the corners to print as far as I can see as long as the lens coverage is adequate.
If I'm missing something here, maybe you could give me a more detailed explanation? TIA