since you wish to calculate the dew point (temperature) in a range of pressures a simple way is to calculate the dew point line or the phase envelope, to do that I use a different software (Prode Properties a simulator which works in Excel and Matlab, as I prefer these tools) but I presume your simulator can to the same.
To evaluate the contribute of water you can calculate the dew line (or the phase envelope) with and without water, that 'll permit to observe the additional two-phases area due to water presence.
To calculate VLE for hydrocarbons plus water take care to select the right thermodynamic model.