In the 3 years that Vaz has caught for the Red Sox, 2014, 2016, and 2017, he has saved the team 58 runs in pitch framing alone, according to Baseball Prospectus. That's roughly 6 games. I'm sure the pitchers are aware of this too, which has to instill a great deal of confidence in them to pitch to him. He has had a rough few games, but he is a very good defensive catcher.
As with JBJ in CF, I'm good with Vaz not being a heavy hitter. Whatever he gives us offensively is gravy.