But, the only similar Vivitar I'm familiar with in that range with Autofocus is the Vivitar Series 1 70-210mm f/2.8-4 APO Macro Autofocus Lens (it's probably a rebranded Cosina). You can find dimmer Vivitars with AF in that range, too (down to f/5.6 on their long end like most lower grade lenses)
and I bought one of these Vivitar 70-210mm f/2.8-4 lenses. I got mine for $79.95 from CametaAuctions on Ebay (brand new in the box with warranty).
I posted some photos from it in this forum thread:
Mother's Finest at Tybee Island, GA
But, my guess is that the lens in question is an MC mount manual focus lens, and the seller is probably mistaken about the AF part.
P.S. If the seller has a photo showing the lens mount, you could tell that way.