enlarged as in view at 100% or more?
on your camera's LCD or computer LCD?
to prevent vignetting caused by the characteristic of the lens, stop down your aperture (different lens will have different apertures when vignetting improves)
to prevent diffraction, dont shoot like very small apertures like f18-f22 (different lens will have different aperatures where diffraction kicks in)
technically speaking, this is the flaws of the lens design, or mistakes during creating the images, we should avoid it.
some time, we can use it to enhance our photos, however, not all images work well with vignetting.