The effect of vapor pressure on the thermoreversible gelation in polycondensation reactions is theoretically studied on the basis of the lattice theory of polymer solutions combined with the conventional Flory-Stockmayer theory of gelation. In polycondensation systems, concentration of the produced three-dimensional branched polymers in the reaction bath changes as reaction proceeds due to the byproduct of the solvent molecules, such as water, alcohol, etc., and hence it is not a controllable parameter. We calculate the polymer concentration as a function of temperature and vapor pressure of the solvent, and we study how sol/gel transition interferes with macroscopic phase separation in such reactive solutions. We find a new reentrant sol phase at high pressure that is brought by a strong backward reaction. The thermodynamic conditions to reach the gel point without phase separation are found. Gel point can be reached even for reactions in closed vessels where pressure changes under a constant volume. The pressure-temperature phase diagrams for closed systems showing sol/gel transition lines and phase separation lines are derived.