Not sure how you missed it because I don't think the Vagary can commit suicide in any way. Must have just glitched, I think I recall hearing some people say that they didn't unlock.
I would simply suggest replaying the game, rushing through the first parts until you get there and then make a save before you fight it. Complete the fight and if it doesn't unlock, keep loading until it does.
I have moved to PC gaming, so I
won't be on Xbox LIVE anymore.