He doesn't make bad movies often. But his general aesthetic and approach to the X-Men has turned off many from the start. People like to use to blame Fox for that, but his last two films have shown that to just not be the case.
First Class focuses on Charles and Erik. It also explores their relationship and actually gives their opposing philosophies some actual footing. It isn't just something that happens because that is the way it is. Their mythic chess game is actually a thing. With two well formed characters who have a geniune relationship, that is bigger then anything Singer had brought to the table in that regard. Who spends most of his time playing up the loner aspect of these characters (Rogue and Logan), and forcing drama ala Jean and Logan.
And Vaughn does a lot with the other characters in their big scenes. Stuff like the kids' naming themselves, the push and pull of Charles and Erik on Raven, the training montage, etc. His directing and editing skills allowed for him to get more out of these scenes then most director's would. It isn't just a training montage, it is a character piece for Hank that sets his relationship with Charles.
Also Vaughn gets credit for simply having an X-Men movie where Charles actually teaches some kids something. 4 movies with Singer, and he apparently doesn't realize that Charles is a teacher to a group of mutant badasses who need assistance in controlling those powers.
