Williams, judging by the Mayorga fight( when Vernon was in his prime) he doesn't react well to pressure from a naturally bigger opponent that is willing to take his best punches and dish them back out even more. Not only that, but Williams is much stronger, much more skilled and much faster( and better) then any version of Mayorga. If Vernon boxes carefully, then he could make it the full 12. But if he tries to brawl, then he would be sent home early by Williams.
this is not the fight that would establish Jr. MW surpremacy. It would establish the "United States Jr.MW champion"
Williams smothers him. Forrest has trouble pulling the trigger these days, and hasn't had a legitimate win against a B fighter or better in about 5 years.
I'd go with Williams, and would add that they ought to hold this fight in the Southeast. Considering both fighters are Southern, there would probably be a market for it. I would like to see Williams perhaps develop a local fanbase.