First, you have to look at the definition of "protagonist". According to our dear pals at Google, the word protagonist can be defined in many different ways.
-the leading character or one of the major characters in a drama, movie, novel, or other fictional text.
-an advocate or champion of a particular cause or idea.
"a strenuous protagonist of the new agricultural policy"
I don't know about you, but when I look at Billy Pilgrim, I don't consider him "leading" in any way. He kind of gets dragged along through his life without a care. If you take Vonnegut as a character in the first chapter (after all, it is a chapter and not a prologue), and you say that the rest of the story is a roundabout way of Vonnegut, the character in chapter one, telling his story of Dresden, then I think a strong case can be made advocating Vonnegut as the "leading character" in the novel. Sure, he doesn't appear as much in the novel as Billy, but should that disqualify him as a protagonist.
I also believe that if the latter definition is used, then Vonnegut, if not the protagonist of the novel, is clearly a protagonist for the themes that the novel is trying to represent. But maybe the answer isn't that simple. One could also argue that there isn't a traditional protagonist. There are no true heroes in this novel. There are some good people, there are some bad people, but for the most part, they're all just people. And maybe that's the point. It seems pretty clear in chapter one that Vonnegut didn't want anyone to be a hero in this novel because he didn't want to show support for any reason to go to war. He wanted the war to seem childish, and I think he accomplished that goal.
