I'm skipping the vapor barrier, letting it breath is the concept. Much the same as cabins up here, as the bus will be exposed to hot and cold cycling dependant on usage, and weather I want to any moisture to be able to breath as oppose to trapping it.
Condensation will form on the warm side, so if your spray foam has been done well all you have to worry about is a thin isolator between the spars and your ceiling material. Then from there airflow/cycling/exchanging should help regulate the moisture difference.
disclaimer.....random redneck ravings