We have used fluorescence and light scattering measurements to characterize the unilamellar vesicles formed in aqueous solution from binary mixtures of the cationic surfactant didodecyldimethylammonium bromide (DDAB) and the anionic surfactant sodium dioctylsulfosuccinate (Aerosol OT or AOT). We focus our study on the early phase of catanionic vesicle formation after surfactant mixing. With two complementary fluorescence probes, the anionic fluorophore TNS (2-(p-toluidino)naphthalene-6-sulfonate) and the neutral probe prodan (6-propionyl-2-(dimethylamino)-naphthalene), we observe the creation of a mixed vesicle via the rapid incorporation of the anionic surfactant AOT (present as surfactant monomers in solution) into DDAB vesicles. Deconvolutions of the overall probe fluorescence emission spectra differentiate among the net surface charges on DDAB-AOT vesicles of varying composition. Furthermore, when TNS is added to DDAB-rich mixed vesicles, we observe time-dependent fluorescence variations that are consistent with transbilayer movement of AOT into the inner leaflet of the vesicle. Additional light scattering measurements reveal composition-dependent vesicle hydrodynamic radii and a time-dependent increase in size for cationic-rich vesicles only.