But Vivitar did dumb down when they started selling non S1 lenenses and the quality of these did vary very considerably, from good/almost up to S1 standard to quite average.
I am not all that familiar with the history of Vivitar, but I am somewhat familiar with what the company offered over the years. I became interested in photography in the early 80s, and by then Vivitar had essentially two lines of lenses -- the S1s and the "regular" ones.
It appears that you're suggesting that the S1 lenses predated the others? If so, I wonder about this. I used to own one of the quite rare Vivitar "Professional" 135mm f/1.5 T-mount preset lenses. This lens dated back to the 60s, I suspect, and predated the S1s, or so I had always thought. Vivitar also marketed a number of preset T-mount refractor telephotos ranging in focal length from 400mm to 800mm. These were big, beefy, all-metal-and-glass optics (not even rubberized focusing barrels) that were marketed in the 60s and 70s, from what I understand.
The Vivitar S1 range hit Tamron and Soligor quite hard in the UK, Tamron fought back by intoducing the SP range equally as good as the S1 range.
Soligor lost their dominant position in the UK as a 3rd party lens supplier & never recovered.
Over here in the States, the S1s were duking it out with Tamron's SP and Tokina's AT-X. Sigma was a latecomer, but they began offering a premium product as well, once AF started becoming popular. Soligor I think just ran out of steam and dwindled away. On the advice of a friend, I bought a Soligor 85-300 two-ring zoom back in the mid-80s, and was totally underwhelmed by its performance.
My impression with Vivitar was that it seemed that the company just lost interest in competing once AF lenses emerged. Vivitar was either unable or unwilling to do all the design work involved in order to supply lenses in a widely varying array of AF mounts and technologies.
Best,
Michael