Fresh water freezes at 0ºC on the surface of the earth (mp on the diagram.) It doesn’t matter if you are in Arizona or Ireland. It doesn’t matter what the partial pressure of water is in the air (i.e humidity.) The freezing point is unchanged.
Yet we know that there is still vapour in the air at minus 10ºC. How can this be? The phase diagram shows no vapour at mp, and certainly not to the left of mp.
The reason we have vapour in the air is because some of the molecules of frozen water are constantly being raised to higher energy states through collisions with photons, and collisions with other molecules.
The temperature can be well below 0ºC, and yet there is no frost on the car. That is because there are just as many molecules being sublimated as there are being frozen. Thus there is no net buildup of ice.
Now suppose the humidity increases, or the temperature decreases to a point where more water molecules are freezing out of the air than are being sublimated. Then we start to see accumulation of frost. That is called the dew point.
If you are clever, I think you will see where I am taking this discussion.