Just my opinion, obviously, but he doesn't really have the pitch recognition ability to excel at the MLB level. He's never hit righties well, and he clearly has no command of the zone. His ability to react to breaking balls just isn't there. This happens to many players, the extent of their physical ability (fast twitch, hand eye ability) isn't good enough. Add in that he lacks the leverage to drive the ball consistently with launch angle when he does get something he can handle, and you have a below average hitter. I saw him play a couple times at single A, and even though he has a couple well struck balls I was nervous about him then. He didn't stand out in any way, struck out a couple times, didn't hammer hitter's pitches consistently.
It's a shame, the first thing is something it's very difficult to scout for, something like the processing ability of a QB going up levels. But Vaughn also just isn't really an athlete, and athletes are going to have a better chance to adapt.