Let me take a stab at the "vapor versus gas" question.
*) There is no physical or chemical difference between a vapor and a gas.
*) On the other hand, the words differ in connotation, and are not quite
used interchangably. The situation is loosely analogous to "beef" versus
"cow". You wouldn't want to sit at the dinner table and ask for another
serving of cow.
*) ISTM the word vapor is used to emphasize that we are talking about the
gas phase, not the corresponding liquid. Examples:
--) They fill balloons from a tank of helium gas. It would sound quite
weird to call it helium vapor. Explanation: there is no liquid anywhere in
sight.
--) Welders use acetylene gas. Same explanation.
--) In the lab, you can produce temperatures below 2K if you start with
liquid helium and pump the vapor away fast enough. Explanation: it's
called helium vapor, to emphasize that it's not the nearby liquid.
--) Congregations of cosmic H20 molecules are called interstellar water
vapor. Nobody ever speaks of water gas. Explanation: water is presumed to
be a liquid unless you say otherwise. These particular H2O molecules are
nowhere near saturated vapor pressure, and have at all times since t=0
remained very far from the liquid state.
--) If you smell fumes in the garage, you call it gasoline vapor, not
gasoline gas. Explanation: again, gasoline is presumed to be a liquid.
Note that the term vapor applies even though the partial pressure is
nowhere near the saturated vapor pressure.
--) I've seen phase diagrams etc. suggesting that anything above the
critical temperature and/or pressure should be called a "fluid" to
emphasize that in this region there is no meaningful distinction between
liquid/vapor/gas. But this is not an if-and-only-if proposition; there
are lots of fluids that are not above Tc or Pc.
*) I changed the Subject: line because pedantry is not important. Clarity
of thought is important. Clarity of communication is important. Pedantry
is an obstacle to both.