A solvent annealing-induced structural reengineering approach is exploited to fabricate polymersomes from block copolymers that are hard to form vesicles through the traditional solution self-assembly route. More specifically, polystyrene-b-poly(4-vinyl pyridine) (PS-b-P4VP) particles with sphere-within-sphere structure (SS particles) are prepared by three-dimensional (3D) soft-confined assembly through emulsion-solvent evaporation, followed by 3D soft-confined solvent annealing upon the SS particles in aqueous dispersions for structural engineering. A water-miscible solvent (e.g., THF) is employed for annealing, which results in dramatic transitions of the assemblies, e.g., from SS particles to polymersomes. This approach works for PS-b-P4VP in a wide range of block ratios. Moreover, this method enables effective encapsulation/loading of cargoes such as fluorescent dyes and metal nanoparticles, which offers a new route to prepare polymersomes that could be applied for cargo release, diagnostic imaging, and nanoreactor, etc.