And the winner is..
Well, there is no clear winner - both would serve very well. The (R)ikenon is a touch smaller overall but the (V)ivitar at 28mm is shorter. Miminum focus at all focal lengths goes a bit closer with R but both focus 'macro' at the same distance, the R at 1:4ish @135mm and the V 1:5ish @105mm. I cannot judge color cast since auto-WB was set, but R was a touch warmer. A bit of flare from the bright window behind my yard-stick, slightly more visible on R. All told it's too close to call - but since V uses the Pentax A mount it's the winner. Due to its design, I can use a step-down ring on the nose and use my 67mm cPL with no apparent vignetting at 28mm f/4. It may be there, but if 28mm f/4 doesn't show it in a cloudy sky it's not worth the worry.. and software could fix any that decides to show up in the future.