Cost-effective production of flexible organic memory devices requires ambient solution casting of ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). However, condensation of water vapor from the environment into the drying fluid film causes phase separation, which yields a fluid state morphology comprising polymer-rich micro-droplets in a solvent–rich medium. As a consequence, the dry polymer films are rough, which leads to a low production yield of working devices. Whereas this process of vapor-induced phase separation (VIPS) is used advantageously in the production of microporous polymer membranes, it is hence to be avoided in the manufacture of thin-film organic memory devices. Through microscopic analysis, device characterization and numerical calculations we identify a processing window for smooth films, determined by the solvent’s evaporation rate and hygroscopicity, as well as the ambient humidity. Modeling of the multi-component out-of-equilibrium phase dynamics demonstrates how microstructure and feature sizes emerge during simultaneous solvent evaporation and water condensation. The numerical simulations yield morphologies consistent with experimentally observed structures and demonstrate how domain size and phase composition depend on the relative humidity of the environment. Interestingly, calculations and experiments seem to support a scenario wherein the dominant feature size in the dry polymer film is to a large extent already determined in the early stages of demixing.