But the whole point would be to throw people off a bit. Rather than ruining the reveal in Episode V, the idea is that it might even be more shocking to find out who Vader is if we have been mislead (though there would be clues along the way, and foreshadowing, but not obvious enough that one could easily figure it out).
EDIT: And the point would not be to confuse people, but to mislead them, until they have a moment of "now everything makes sense--I can't believe I didn't figure this out before" after the surprise of the reveal in ESB.