Need WriterVol to teach us to English on this one
Vols fans vs. Vol fans (wall o' text edition).
My personal preference is that is just doesn't matter.
You find both cases in use among fans and even among journalists. I'd say go ask a journalist, but I'm not sure there's even an AP Style rule on it (not that many journalists these days bother consistently applying a rule set). I didn't find anything on a rule in very brief online searches or in my old AP Stylebook.
Even utsports.com doesn't consistently use one or the other, which tells me they don't have a set rule (or they have bad editors). For the Tennessean, I only found "Vols fans." For the Knoxville News Sentinel, I found both uses. Various other sports news outlets use a mixed bag. Again, this is using brief searches.
On a technical level, I feel like Vols fans is more correct, but Vol fans seems to sound better to me in many cases. Having back-to-back words ending with an "s" takes more effort in speech so it's easy to see why many would drop the "s" on Vol.
You even see examples of people saying "Bear fans," though it's much less common. That double "s" takes effort. Effort often gets thrown aside in speech.
As an editor, I would tell a writer--in the absence of a set rule--to decide on a rule and apply it consistently.
Is it any wonder English is a difficult language?