I'm not necessarily defending EVH about his playing on that gig but just as a way of possible explaination. Most of his career is playing music that he wrote. Therefore, all his soloing chops is mainly geared towards, and worked out against, those progressions and the trademark VH style.
So I can see where playing up against an unfamiliar style and progressions might not work so good for him.
So I can see where playing up against an unfamiliar style and progressions might not work so good for him.