Like most fast lenses, this Vivitar 28mm f2 (Komine version), vignettes pretty severely wide open. However, it's basically gone by f4. One of the nice touches of this lens is the half-stop settings. You can set half stops from f2.5 to f11, and between f11 and minimum aperture of f16, there is no half stop.
Vignetting Test from f2.0 to f3.5. Please excuse the dirty sensor. Larger Picture.
Vignetting Test from f4-f8. Please excuse the dirty sensor. Larger Picture.
In terms of flare, it didn't fair too well wide open, but improves quite a bit as aperture closes down to about f4. I did not use a lens hood. It might have been better if a hood was used. To be honest, this was expected as most older lenses were not very good at flare control, even with multi-coating, as in this one. But, this kind of condition is tough on even the best of lenses.
Flare test wide open at f2.0. Larger Picture.
Flare test at f4. Two stops down, must better. Larger Picture.
Next up, sharpness test.