This is probably the best copy of a Vivitar fast wide angle lenses that I have had, both in terms of optically and cosmetically. I did a test of this lens, and you can find the blog entries here:

Part 1: http://oldlenses.blogspot.com/2009/12/vivitar-28mm-f2-for-nikon-ai-s-mount.html
Part 2: http://oldlenses.blogspot.com/2009/12/vivitar-28mm-f2-for-nikon-ai-s-mount_31.html
Part 3: http://oldlenses.blogspot.com/2010/01/vivitar-28mm-f20-part-3-vignetting.html
Part 4: http://oldlenses.blogspot.com/2010/01/vivitar-28mm-f20-part-4-sharpnesstest.html