I read somewhere about some panel van diyers waxoyling interior panels.... doubt I could find time for that myself but imagine that it might help as long as bottom drainage from sills isn't blocked in the process.
Dacron looks like a reasonable option glued to the back of ply lining or somehow curtain floated.
For anyone feeling flush thinsulate blanketing is available for floating/curtain type attachment.
I reckon that unless you can truly 'bond' a material to the steel panels then it's better to leave breathing room.
Hanging Vapour barriers will have some, possibly considerable, effect but are unlikely to be efficiently moisture proof when applied to a panel van.
I forgot about dacron.... It looks like a pretty good compromise to me....and is utilised by professional converters.