(SPOILER)
Ignoring the fact Chapter 11 blows it's own first plot twist by having the Datalog tell you that Vanille was Ragnarok, despite never being stated in any of the cutscenes up until that point. The game then says that Fang was actually Ragnarok and Vanille was pretending to be the one who became Ragnarok to protect her, which doesn't really make much sense to me. Its not like Fang really gives a damn about Cocoon and of all the characters, is the most insistent and shows favor in completing there Focus (well until they realize the Fal'Cie R EVIL). So I don't see why she needs to protect her or anything from the truth.