You're in the right section, and welcome!
I ran across this lens about a year ago and couldn't find much, but I did come to suspect that it's a re-label of the same lens that is also sold as an Opteka:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/56604-vivitar-...50-1300mm.html
I can't confirm it's identical to the Opteka, but I strongly believe it is. Other lenses (such as the reasonably well-regarded 85mm f/1.4) are sold under Samyang, Vivitar, Opteka, Bower, and Rokinon names at the same time, while being the exact same product optically with only superficial external changes to distinguish them.