Answer: With some negligible loss in accuracy(less than 1%), the water vapor in the atmospheric air can be considered as an ideal gas because there is not much difference in the properties of water below 12 kPa (which is saturation pressure at 50 degrees Celsius). It obeys ideal gas relation Pv=RT.
But at high pressures and temperature there can be a significant error in values so at high pressures and temperatures (for example in steam cycles), one should not use consider water/steam as a ideal gas.