An extension of previous work, the study examines the implications of the approximation that the fuel vapor and liquid temperatures remain equal during the transient. Modified mathematical formulations for calculating the transient fuel vapor temperatures separately are provided as well as the results of calculations of the dynamics of fuel vapor pressure buildup during transient heating in voided liquid-metal fast breeder reactor cooling channels by dispensing with the above approximation. The results with and without the approximation are compared with each other. The study indicates that, although the fuel vapor temperatures lag the liquid-fuel temperatures, the fuel vapor pressure buildup is relatively less sensitive to this lag. The use of the above approximation results in an overprediction of the transient vapor pressure by <10%.