I've tried several Vivitar 28's, and had very mixed experiences. This one - the f/2.5 made by Kiron - is one of my favorites, but even there my first copy didn't grab me at all. It's a very distinctive lens, with a widely flared filter ring that seems much larger than the front element would seem to dictate;
https://www.pentaxforums.com/userreviews/vivitar-28mm-f2-5.html has some pics.
My first copy had muddy colors and an overall mushy look. The second copy was bought more-or-less on a whim - I saw it sitting alone on a shelf at an estate sale giving off a forlorn puppy-dog-eyes feeling, so I picked it up without knowing for sure what I'd do with it. My first outing with it was at an auto museum, and it grabbed me with the way it rendered subtle tonal shading; the glossy shade of waxed paint on the VW bus above, or the gleam on the nose of a Lockheed Electra at the TWA Museum at the KC Downtown Airport:
The copy above is one I picked up recently; it was only $10, it would be nice to have a backup, and it was also nice to have a copy in OM mount (the first good copy is in Minolta SR). Haven't had a chance to test the gloss yet, but it did a nice job with the sky.