Guns N' Roses Guitarist DJ Ashba -- Concert Fight ... Over an E-Cigarette! (VIDEO)

Guns N' Roses guitarist DJ Ashba's e-cigarette vapor pissed off a Vegas concertgoer so badly ... the guy put an ass whupping on Axl Rose's axman.

It's not the most rock 'n' roll brawl we've ever heard of ... but here's what went down. Ashba was at Planet Hollywood over Memorial Day weekend to see Romeo Santos perform. Our sources say when the guy said something about the e-cig ... Ashba shot back, "What are you going to do about it, buddy?"

That question sparked the fight, and we're told the guy's girlfriend even got in the mix.