Vargas was a simple fix. He already had a good approach and good pitch recognition. He then gained 20 lbs of muscle because they wanted him to drive the ball more.
He was still making weak contact however so they identified something in biomechanics that they thought higher hands and flatter (horizontal ) bat pre-pitch would help.
Robert is more complex. He's never had a good approach or particularly good pitch recognition which is why with so much bat speed and physical talent he can look so bad.