I am not sure that the Mr Heater produces vapor or steam. I have 2. Yes
there's condensation on the propane bottle when in use but not vapor from
the radiant heat. I think the interior of the van and other stuff in the
van contain a small percentage of humidity and the heat from the heater
makes the humidity evaporate.
Similar to hanging a wet cloth or a wet towel in front of the heater...
steam will come out of it