I'll make a stab at answering Robert's question. What he has described
as his vapor is what is called a "saturated vapor", a substance in the
vapor state which is, potentially, in equilibrium with a liquid at the
same temperature and pressure. The vapor state is much less limited.
The term vapor also pertains to states of the substance which are at
lower pressures or higher temperatures than the saturated vapor states.
There is a convention which some physicists follow that says the term
vapor only pertains to states which are below the critical isotherm.
(I think Michael said chemists have another convention, but that seems
wrong to me.) One might guess that nitrogen gas at room temperature is
called a gas because it qualifies as such by that convention. I doubt
that; I think nitrogen was probably called a gas before the silly
convention appeared (no doubt in a textbook) at all. The OED says that
vapour is the state of matter to which liquids pass naturally by the
action of a sufficient quantity of heat. That would conform to Robert's
Rules. [sorry, I couldn't help it.]