If a coach's job is to get the best out of his players, then Jeff Fisher mostly at fault here. Because his actions indicated that he was never 100% on board with Vince as his quarterback.
That doesn't mean that Vince Young is without blame. He wasn't accurate, didn't put the work in, and he had an inflated image of himself when he came into the league.
You wonder if maybe Vince Young should have wound up in Arizona and Matt Leinart in Tennessee. Maybe things would have worked out better for the both of them.