We demonstrate that the methodology of frequency-resolved optical gating (FROG) is applicable to time-resolved reflection spectroscopy of a plasma mirror in the vacuum-ultraviolet (VUV) region. Our recent study [R. Itakura et al. Opt. Express 23, 10914 (2015)] has shown that a VUV waveform can be retrieved from a VUV reflectionspectrogram of a plasma mirror formed on a fused silica (FS) surface by irradiation with an intense femtosecond laser pulse. Simultaneously, the increase in the reflectivity with respect to the Fresnel reflection of the unexcited FS surface can be obtained as a time-dependent reflectivity of the plasma mirror. In this study, we update the FROG analysis procedure using the least-square generalized projections algorithm. This procedure can reach convergence much faster than the previous one and has no aliasing problem. It is demonstrated that a significantly chirped VUV pulse as long as 1 ps can be precisely characterized.(Grant No. ZE26B-27).