I'd hardly say Vaughn is an over rated guitarist. The man has miles more technical guitar aspects than Slash has incorporated into the majority of his work. If it comes down to "catchy riffs" then yes, Slash has the upper hand. But when it comes down to sheer guitar skills, Vaughn has the upper hand.
And I'm not getting at him making it big because he was in a band. I'm getting at it not being like a "wow this guy is a great guitarist" rather than "Oh, that's a catchy riff. And ooh, listen to those vocals".
When people listen to Vaughn, Mayer, Hendrix, Clapton, etc. people hear lots of exceptional guitar playing. So it's hardly that he couldn't have made it big on guitar skills alone, look at Clapton. He could have been solely just the guitarist for Cream, and even if he hadn't had a solo thing people still would have cared about him as a guitarist.