Regarding vapor barrier: We have everything sealed up with foil tape. Almost 2 years of fiberglass batting and spray foam in our van, zero problems with moisture being held in (as far as we can tell).
I mean, if moisture is that much of a concern, you would have to also worry about the van seats too, being giant foam sponges, and I haven't seen anybody worry about those.
The key is to try and mitigate moisture build up by opening windows or vents, and when moisture does build up, air out the van during the day and let everything dry.