If the batting coach doesn’t have anything to do with the Jays performance. That they hit great in April and went into the dumps in May. Did he all of a sudden turn into a bad coach - no - but maybe he has lost there ear or isn't preparing them and helping them to make adjustments to piching patterns. As you will note - the jays rarely hit to the opposite field as they have in the past.
I don't think a coach at the major leagues can make a player great - but he can prepare him for success and he can adversly effect his performance. Vernon has gone up to the plate without at a plan of attack. He has consistently swung at pitches that aren't pitches he can drive or hit for a hit, essentially pitchers pitches. Often doing this with runners in scoring position - this is all about preparation. Making sure you get the right pitch. He is giving up at bats. That is Brantleys fault - and his alone. I would say this has cost them about 5 to 6 games in the standings.
The difficult part of hitting in the middle of the order is that you are hitting with runners on base. The need to be selective is huge.