I always hear him with very feint midwestern accent.
Based on his picture in that one book (a Mission Gamma, I think), I can definitely see that. Of course, my mind will still probably play Sean Connery's voice in my head, just because I think he's really cool.
But I think James Brolin or someone similar would work well, too.
What am I talking about? I haven't even read any Vaughn books yet!