Polymer vesicles are being extensively studied to emulate self-assembly in biological systems and also use them in a variety of biological and industrial applications. This study demonstrates a novel strategy to prepare polymer vesicles in a pure aqueous medium by driving the micelle-to-vesicle transition with metallic nanoparticles. We synthesized poly(2-amino-2-hydroxyethyl aspartamide) (PAHA) substituted with octadecyl chains, which could form micelle-like self-aggregates in the aqueous medium and chemically bind with platinum precursors. Then, in situ polymerization of Pt nanoparticles within the PAHA self-aggregates generated polymer vesicles that possess nanoparticles within bilayers, because of the increase of the hydrophilic mass ratio to total mass of PAHA, f(w). This new strategy to prepare polymer vesicles would greatly serve to facilitate the control of self-assembly and ultimately improve the functionality of a wide array of polymer vesicles.