Complaining about platoons/splits is a universal trait of baseball fans. Fans always want to play the hot hand, most teams these days play the numbers. I think Vaughn may have gone cold from lack of play, but there clearly was some injury-related element that may have improved post-ASB.
As for Yelich, I honestly don't care. He has league average wOBA, a .330 OBP, and is a plus baserunner. There are worse options for leadoff hitter.