Dielectric surface flashover in vacuum is characterized by a three-phase development, as shown by current measurements covering the range from 10-4 to 100 A, assisted by x-ray emission measurements, high speed photography, and time-resolved spectroscopy. Further information is gained from a comparison of the flashover dynamics at 77 and 300 K. Phase one comprises a fast (several nanoseconds) buildup of a saturated secondary electron avalanche reaching current levels of 10 to 100 mA. Phase two is associated with a slow current amplification, with a duration on the order of 100 ns, reaching currents in the ampere level. The final phase three is characterized again by a fast (nanoseconds) current rise up to the impedance-limited current on the order of 100 A in this specific apparatus. The development during phase two and three is described by a zero-dimensional model, where electron-induced outgassing leads to a Townsend-like gas discharge above the surface. The feedback mechanism towards a self-sustained discharge is due to space charges leading to an enhanced field emission from the cathode. A priori unknown model parameters, such as field enhancement factors, outgassing rate, and the buildup of the gas density above the surface, are determined by fitting calculated results to experimental data.