I use FF3 to size bursting disks and safety valves for fire scenarios. One useful feature would be if I could set the vapor fraction and pressure for an assigned pressure boundary, and let the program calculate the temperature. What I normally do in those cases is calculate the saturation temperature apart (in Hysys for example) and then add a few extra degrees to guarantee it will be in vapor phase (sometimes if I use the exact calculated temperature from the process simulator it doesn't work in FF3 (still remains in liquid phase), obviously due to different equations of state used in each software).
I hope the above is clear. Sorry for the long post!