If there is pure water vapour in the gas phase, can the partial pressure be taken to be equal to the vapour pressure of water at that particular temperature? edit: How is relative humidity calculated for pure water vapour?
The nature of your post seems to indicate that you've conflated some different things. Let me just sort them out and I think you answer will be clear.
Vapor pressure of water
Just the partial pressure of the water vapor, which may or may not be at equilibrium.
Equilibrium vapor pressure of water
For a closed isothermal system, with a gas phase and a liquid phase and between 0 C and 100 C, there is a particular vapor pressure of water that exists for the system at equilibrium. In general the higher the temperature in that range the higher the vapor pressure of water at equilibrium.
The vapor pressure of the water vapor does indeed depend on the purity of the water. So at a given temperature the vapor pressure of salt water would be less than that of pure water. Also if there is a solvent mixture of water the vapor pressure water will be less than that for pure water.
For a given temperature there is a maximum amount of water (vapor pressure) that can exist in the vapor phase. Any excess will condense. So the relative humidity for a given temperature is expressed as the percentage of measured vapor pressure as compared to the maximum vapor pressure over pure water.