dave85374 Attached is a photo of M31 which definitely shows the problem.
Are you conflating the problem of intensity distribution that is caused by aperture vignetting, with the intensity distribution caused by light distribution from a galaxy?
It seems your problem is the latter (i.e., bright galactic core vs its arms), and has nothing to do with aperture vignetting (which flat frames solve).
The way to see more of the galactic arms is to stretch the intensity (what some people call "stretching the histogram").
Chen