Does the back of the lens look like this?
Some Vivitar lenses have a very long flange which is completely unnecessary. It bangs into the part inside the K-mount where what were the power-zoom contacts (now focus motor contacts) are. These lenses can be mounted on manual focus SLRs fine but can damage AF film or digital SLRS.
I removed the metal piece and cut it down to what you see below. The back piece can be just removed but that leaves the aperture lever unprotected and the back of the lens open for dust to enter.