Upload & Sell: On
I'm guessing this is inherent to the lens. I don't have the 21/2, but I do have the 18/3.5 and it retains a vignette at all apertures as well.
The Oly's (imo) trade the excellent distortion for the vignetting ... just the nature of optics / physics of light, those who design less vignetting, will necessitate more distortion (or weaker corners) ... the wider you go, the tougher it gets ... choose your poison.
I've got both the Oly 21.3.5 and the Nikon 20/2.8 AIS, but neither are available for me to use atm ... but from memory both have more reasonable vignetting than your 20/2 is showing @ f11.
I haven't gotten around to it yet, but I figure I should make a vignetting action specific to each stop @ my Oly 18/3.5
f8, f11 & f16 from a test set some time back.