You know, I'm pretty sure there is something wrong with my copy, and it's probably too late to do anything about it.
I don't have it with my here, but I found a couple examples of it:
If you look at the EXIF, they're both 75mm, f2.8, ISO 800, 1/125s. I realize these are not ideal testing conditions, and it honestly is hard to believe that isn't just out of focus and/or motion blur. However, that same evening I went out with my 100mm and took some pictures with the same settings, and they were only very slightly motion blurry, and were in focus (I was manual focusing with both).
2 years ago the lens rolled out of my camera bag onto cement, and got some minor dents on the plastic (the back cap, tiny one on the aperture ring, and I think the lens hood). I don't THINK this was the cause though, because I know I didn't like it before that as well. Perhaps it made it worse though.
EDIT: I just searched for older photos, and found that wide open photos from before that are nowhere near as bad. They're not as good as my Vivitar of course, but they're not too bad. I got the lens around 4 years ago I think, from B&H. This suggests it's my fault, so even if it were still under warranty it probably isn't covered... any ideas on how much it would cost to fix?
EDIT2: Here's
some photos before the drop, and all the
photos from after. I think it's very clear it's shifted, and I feel silly for not realizing this earlier...