Originally posted by kdermott Vivitar 75-205mm 1:3.8 Macro Zoom Lens
A couple of points:
1) Vivitar made lenses for many different mounts so you need to make sure the particular one you are looking at is for Pentax.
2) The "Macro Zooms" of that era are not real macro lenses by any means. Putting 'macro' on the lens was mostly marketing. They do usually have a 'close focusing' mode that is useful, but it is not macro.
3) As already noted some Vivitar lenses have an oversize aperture guard that will not work with Pentax digital cameras. This can usually be removed or modified so it will work however.
4) This lens has no 'A' position on the aperture ring so it will be all manual and you need to use stop down metering.
Here is the listing for what appears to be the lens you are looking at:
Vivitar 75-205mm F3.8 Macro Focusing Lens Reviews - Vivitar Lenses - Pentax Lens Review Database