Definition of enthalpy of vaporization
Assuming that the reservoir does not absorb any heat and there is no loss of heat from the setup by:-

Radiation of heat from the reservoir,
Convection currents in the air/surrounding fluid (if any),
The entire water is at the same temperature so that heat absorbed is not used for increase in temperature,
Conduction loss from the reservoir to the ground/support it is on and air(negligible but thought I should mention)

Fulfilling these conditions ensures that the energy is only used by the water for vaporization.
So, yes, these conditions are fulfilled. But, that is never the case when experiments are conducted in labs.
Edit: The above conditions are more than just an isolated system.