alright, I did some internet searching and actually found discussions related to free energy of vaporization....with actual values. Yes, I assumed that dS(system)=dH/T, assuming that at each stage of vaporization, the system is at equilbirum, making it a reversible process (which I thought was so). Interesting.
You may actually want to plug in actual values, for instance, find the entropy and enthalpy of vaporization (CRC or NIST, scifinder) and with the respective temperatures, plug into the dG formula.
Another possibility is that at one atm, and at the corresponding boiling point, the free energy of vaporization is zero, but at temperatures otherwise, it may not be zero, since the chemical potential of the liquid and gases differ respectively.