Hi Photolady,
I know this doesn't exactly answer your question, but you might be interested in this thread;
Cosina 100mm F3.5 MC Makro Lens Reviews - Cosina Lenses - Pentax Lens Review Database
The Cosina-made 100mm f/3.5 Macro (Makro) appeared in at least four badgings, Cosina, Pentax, Vivitar and Promaster. Same optics, same plastic outer shell, slightly different cosmetics like lettering, DoF scale and of course brand name. Several of the posters on the Review link mention that they have the Vivitar version, and you can see the score is pretty high for a (basically) budget, 3rd-party-made, lens.
I own the Pentax-branded version and have been very happy with mine.
https://www.pentaxforums.com/lensreviews/SMC-Pentax-FA-100mm-F3.5-Macro-Lens.html Yes, it's only 1:2 and the plastic housing doesn't exactly inspire confidence for durability. Pentax also made an f/2.8, 1:1 100mm Macro in this series (FA) and for three times the price, is certainly a higher-quality macro. But this plastic fantastic (as the Cosina was called at the time) is no slouch, I've recommended it many times as one of the best under-$200 AF macros available.
I believe there's also a manual-focus version of this, too. Mine is A/F and has a real aperture ring, very helpful for macro.
JMO, hope this helps!
Ron