Is water vapor in a metastable state at 1 atm and at room temperature? At 1 atm and room temperature, is water vapor in a metastable state? As I understand it, water vapor is water in a gaseous state when the stable phase of water under this temperature and pressure is liquid, not gaseous. 
I understand that water vapor can be the result of evaporation, where water molecules near the surface water-air can get enough energy from thermal fluctuations to escape the attraction by other water molecules from the liquid and go freely into the air. It is also my understanding that when the air is saturated in water vapor, the rate at which water molecules escape the liquid is equal to the rate at which water vapor molecules condensate into the liquid but that is beside the original question I believe.
If the answer is yes, does this mean that thermal fluctuations help the system to reach a thermodynamics equilibrium which corresponds to a coexistence of a stable and metastable states of water as opposed as to a stable state of water only? If so, then I don't really understand why the thermodynamcis equilibrium does not imply a stable state only, but forces a metastable state to exist. I would like some explanation here.
If the answer is no, does this mean that in the system liquid water + air, the thermodynamics equilibrium corresponds to saturated air with water vapor as well as a liquid phase (assuming there was enough liquid water to saturate the air), so that the water vapor pressure in the liquid matches the one of water vapor in the air. If so, then I don't really understand what temperature-pressure phase diagrams of water represent. They would seem not to necessarily represent the equilibrium state of water under the indicated T and P. Or maybe they do, but are valid only for a system entirely composed of water and not water + air. This last guess seems to be correct, I am right?
 A: Metastable water vapor exists as supercooled vapor or in supersaturated air e.g. in the absence of cloud condensation nuclei.
At room temperature and a pressure of 1 atm supersaturated air exists, but I doubt that it is possible to prepare pure supercooled water vapor at this conditions.
The usual phase diagram of water describes water as a pure substance, not as an n-ary mixture with gases. According to this phase diagram, water is liquid at $20\mathrm{°C}$ and $1~\mathrm{atm}$ ($1013.25~\mathrm{hPa}$).

However, when you follow the line $T = 20\mathrm{°C}$, you will find an intersection with the curve that separates the vapor and liquid phase. The pressure at this point ($23~\mathrm{hPa}$) is called the vapor pressure at $20\mathrm{°C}$. This is approximately the same as the partial pressure of water in saturated air at the same temperature. So you can think of air molecules as mere spectators that do not contribute to the "water pressure".
Conclusion:
The answer would be no under the additional assumption that the air is saturated with water vapor. However if air is supersatured with water vapor then the water vapor would be in a mestastable state and the answer would be yes.
A: A liquid necessarily requires many water molecules to collect together.
You could ask how many are required before it is considered a liquid? 
If the answer is "more than one", then yes, water vapor molecules in the air are colliding with each other all the time. 
Only when the conditions of the collision lead to combination due to Hydrogen bonds is a liquid formed. 
These bonds can exist momentarily prior to being broken - in which case the liquid became a gas again. I'm not sure what you mean by meta-stable in this context, but it could be described as bi-stable. 

Behavior of water in air is indeed different from in a vacuum.
A vacuum can be empty space and therefore low pressure, but if water vapor is compressed and cooled in a steel box then a higher pressure can be achieved in the absence of air, and it will liquefy.
Ideal PT diagrams for water should be in the absence of air. It's a fair approximation to behavior in the presence of air.

Note is actually quite a special liquid (or maybe we just spent more time studying it). From the first observation that the solid floats on the liquid it continues to deliver mysteries.
