We study the self-assembly in thin films of a lamella-forming polystyrene-block-poly(2-vinylpyridine)-block-poly(tert-butyI methacrylate) triblock copolymer by means of scanning force microscopy (SFM) and cross-sectional transmission electron microscopy (TEM). We follow the time development of the microdomain structure in the films during solvent vapor treatment. After preparation on a polar substrate preferentially attracting the middle block of poly(2-vinylpyridine), the films exhibit a spongelike microdomain morphology. With subsequent solvent vapor treatment, metastable lamellae with the spacing much smaller than the equilibrium value are first formed from free surface and grow toward the substrate and eventually establishes a multilayered structure throughout the film. As a result of this process, defects associated with the lamellar structure are annihilated from the free surface and accumulated close to the solid substrate. On an extended solvent vapor treatment, the lamellae thicken toward the equilibrium value. This lamella thickening is accompanied by autophobic dewetting of the thin film.