What Code/standard is the pressure vessel? What is vessel's MAWP? I'm most familiar with ASME Code, which allows 21% overpressure in fire scenario. For example, if MAWP = 64.4 barg, then maximum fire scenario sizing pressure = 77.9 barg (64.4 x 1.21) under ASME Code.
No, all water has not vaporized at 64.4 barg and 281oC. Not at the instant the RD bursts. Inside your vessel is two phases at 64.4 barg and 281oC.
The relief system (RD, pipe, and fittings) will restrict the water vapor to a certain flow rate given the 77.9 barg sizing pressure and the coincident temperature of the boiling water in the vessel. Let's call this maximum flow rate = X kg/hr. Think compressible flow. Choked flow.
The heat input from fire will generate a certain flow rate of water vapor. At 77.9 barg and 288oC, the latent heat of vaporization is about 1489 kJ/kg. Let's call this generation rate = Y kg/hr.
Your job is to provide a relief system such that X > Y.