Micelle-vesicle-micelle (MVM) transitions are observed in the aqueous-mixed ionic liquid (1-butyl-3-methylimidazolium octyl sulfate and 3-methyl-1-octylimidazolium chloride) system. The surface activity of mixed ILs, phase behavior, and solution structures in the system have been thoroughly characterized using conductometry, tensiometry, fluorimetry, dynamic light scattering (DLS), viscometry, turbidity, atomic force microscopy (AFM), transmission electron microscopy (TEM), and (1)H NMR techniques. Synergetic interactions between the two ILs in monolayers at the air/water interface and in micelles/vesicles have been determined using the regular solution approach, and the origins of spontaneous vesicle formation in this novel system are discussed. Using a photoreduction method, the formation of stable gold nanoparticles (GNPs) and microscale nanosheets of different shapes and sizes in the micellar and vesicle solutions has been reported. The studies show the potential of a mixed IL system in constructing stable micelles/supramolecular assemblies, such as bilayer vesicles, which are effective in the preparation of the desired nanomaterials.