I never thought of "You must confront Vader" as a right of passage. I think Yoda just meant that Luke had to overcome his the fears and feelings that have been clouding his judgement before he could truly be a Jedi, and that Vader was a living embodiment of those fears and emotions. By confronting Vader, he is confronting the conflict within himself, and only by doing so, can he truly be a Jedi. It could also be interpreted in a less symbolic and more literal way by reasoning that Yoda meant that as long as Vader and the Emperor are in power, the Jedi would never rise again. I prefer the metaphorical, symbolic interpretation, though.