Many versions of various Vivitar lenses — and other brand lenses made by the same companies that built Vivitar lenses — have an issue that causes problems with many Pentax cameras: A flange which is meant to protect the diaphragm actuation lever but which is much bigger than is really necessary. If your particular lens has this "feature" it will prevent the lens from being mounted on many Pentax cameras, including all the digital SLR's.

Fortunately, there's a fairly easy fix: Just cut or grind away the excess material as shown in the image below. Obviously (or at least I hope it's obvious!) you should remove this baffle with the protective flange from the lens before you begin cutting or grinding. You don't want filings dropping into the inner workings of the lens and you certainly don't want to slip and damage the lens. Luckily, this piece is held in place by just three or four small screws that are easily removed.