We report a quasichemical model for point defect equilibrium in Cu2ZnSnS4 (CZTS). An ab initio calculation was used to estimate the changes in the phonon spectrum of CZTS due to trial point defects and further vibrational free energy, which in turn influences the final defect concentrations. We identify the dominant point defects and estimate the free carrier concentrations as functions of the Zn, Cu, and Sn chemical potentials, the sulfur chemical potential being set by the vapor-solid equilibrium with elemental S at the same temperature as the sample (one-zone annealing). As hinted by calculated low formation enthalpies, either the Cu vacancy (V-Cu(-)) or Cu on Zn antisite (Cu-Zn(-)) acceptors are expected to dominate over a wide range of cation chemical potentials. However, the sulfur vacancy (V-S(2+)) becomes a dominant compensating donor especially for one-zone annealing conditions. We also find that different native defects induce distinct perturbations to the vibrational free energy, resulting in non-trivial qualitative and quantitative shifts in the defect equilibrium. At typical annealing temperatures and Zn-rich conditions, this may introduce especially strong modulations in the concentrations of Zn-Sn(2-) and, contrary to enthalpic predictions, of Zn-Cu(+) compensating donors. The modeling indicates that one-zone processing should result in CZTS, which is p-type but extremely compensated because native donor defects are stabilized by the low Fermi level and finite-temperature effects.