Self-assembly of an ABA amphiphilic triblock copolymer into vesicles in dilute solution was studied by successfully combining experimental methods and a real-space self-consistent field theory in three-dimensional space. It was found experimentally that vesicle size was sensitive to the initial copolymer concentration in the organic solvent. Also, the aggregate morphologies and vesicles sizes were found to be dependent on the annealing time. A number of complex vesicles, such as global, long-style, trigonal, and necklacelike vesicles, were obtained in our experiments. Moreover, the corresponding microstructures were produced in our simulations. The results show that various vesicles in dilute solution are formed solely on account of the inhomogeneous density distribution in the local region in nature. Our simulations confirm that the structural complexity coexisting behavior in the single-amphiphile systems is largely attributed to the metastability rather than the polydispersity of the triblock copolymer. These metastable states should strongly depend on the pathway of the system on the free energy landscapes, which is governed by the initial condition.