You are right to be worried about condensation. It is a real problem with us skoolies living in metal tubes.
I personally did not mess with a vapor barrier. For one big reason... a vapor barrier only works if there are "weep" holes at the bottom of whatever you are covering with plastic. (That is how it works in stick built houses) Without them, the water will just pool at the floor and you will have gained nothing. Our BB and I suspect other as well have "weep" holes drilled into the base of all the walls. So whatever water is condensing on the interior of the factory walls, it now has a place to drain out eventually.
Not to mention, everything has a permanence factor and even plastic will allow water to transfer past it after a bit of time. My thought process which very well may be flawed is that I'd rather try to control the condensation that WILL occur no matter what with ventilation. Therefore I'm basically allowing the bus to sweat and just drying it out as quickly as I can.
Ventilation is the key in my opinion. You can't stop all the condensation no matter what you do so moving the air around is second best.
As far as insulation goes, we kinda went sparingly. Mainly because of the windows. There was no way for us to properly insulate them like a wall would be so it kinda defeated or at least severely lessened our R value throughout the entire bus. Basically I did not bother to try and well insulate the bus because of the giant single pane windows spread about.
We went with 1" pink/white foam on the floor, 3" bat on the walls and left all the 1" factory bat insulation in the roof and lower walls. No vapor barriers.