Polymeric vesicle formation is dictated by the mutual diffusion of water into the bulk block copolymer and vice versa. The hydration of three poly( ethylene oxide)-co-poly(butylene oxide) copolymers with different molecular weights has been monitored both macroscopically ( confocal laser scanning microscopy) and microscopically (small-angle X-ray scattering). Both methods have revealed that the amphiphilic block copolymers swell in water following two qualitatively different growth regimes. Initially, water and copolymer diffuse into each other following a subdiffusional growth as the result of a molecular-level arrangement of the amphiphilic membranes that comprise the swollen copolymer. After a critical time, which is exponential in polymer molecular weight, the amphiphilic membranes reach their equilibrium morphology and as a consequence the growth starts to follow Fickian diffusion. The complex hydration kinetics dictate the phases formed at the interface between the amphiphilic copolymer and water. Upon hydration of simple amphiphiles, the amphiphilic film swells and the concentration gradient at the interface with water gradually drops to zero. This strongly affects the complex driving forces that control vesicle formation. Indeed, to form vesicles, an energy barrier has to be overcome, and therefore a constant concentration gradient is required. We show, by enhancing the hydration kinetics via an ac field, how the interface concentration gradient is kept constant and the magnitude of this gradient dictates the final size of the vesicles.