I think you are over-thinking it. There hasn't been any vapor barrier in the walls for the past 50 years, right? In my estimation, the inner skin is your vapor barrier, albeit a little leaky. If you want your interior skins to be an even better vapor barrier, then seal the places where one skin section overlaps the next, and seal the interior pop rivets.
Then again, you might want your wall interiors to be able to breathe a little, considering when the shell leaks, you will end up with a puddle of water inside your wall at the base.