Temperature for PSV orifice sizing.

Use the temperature of the boiling liquid at the relieving pressure (set pressure plus the overpressure, usually 21% for a fire). The vapor is not superheated by the fire since heat transfer from the fire, to the unwetted wall and to the vapor is minor (frankly, if the fire is heating the unwetted wall that much, the vessel is going to fail making the relief valve sizing issue a moot point).

If you have a broad range boiling point liquid, a common practice is to do a 5% flash on the liquid and use the resulting temperature and associated properties (MW, compressibility).