The phase diagrams you refer to make a number of simplifying assumptions in their construction. In particular, they assume you are dealing with pure substances. In other words, the phase diagram for water is correct only if the piston is filled with water and nothing else.

With an ordinary gas, there would be some degree of gas dissolving into the water and some degree of water vapor in the gas. With your special gas, it's easy to see there could be no water vapor: by construction, the special gas always has a partial pressure of 1 atm and a total pressure of 1 atm, so the partial pressure of water vapor is zero.

The phase diagrams you refer to make a number of simplifying assumptions in their construction. In particular, they assume you are dealing with pure substances. In other words, the phase diagram for water is correct only if the piston is filled with water and nothing else.

With an ordinary gas, there would be some degree of gas dissolving into the water and some degree of water vapor in the gas. 

With your special gas, it's easy to see there could be no water vapor: by construction, the special gas always has a partial pressure of 1 atm and a total pressure of 1 atm, so the partial pressure of water vapor is zero. That said, such a gas certainly does not exist in nature.