Polymer vesicles show great promise as both a targetable delivery vehicle and a platform for cellular mimicry. In pursuit of both these goals, it is important to exercise control over the final properties of the colloid, including surface chemistry, surface topology and mechanical properties. Since polymer vesicles are self-assembled fluids, they respond to all stresses imposed by the solution by changing shape and size. The self-assembly process is critical to forming uniform and reproducible surfaces on the particles, but limits the conditions in which the particles can be utilized. To stabilize the vesicles and introduce a structural element to the particles that can be both solution responsive and tunable, we confined hydrophilic monomers inside the polymer vesicles and polymerize them in situ. The crosslinked particles are the same size as the starting vesicle population, which can be tailored from several hundred nanometers to tens of microns. Since the resulting particles have the same surface as the original vesicle population, this approach allows the mechanical properties of the particles to be tuned independently of the surface chemistry and bilayer properties. By varying the monomer composition and crosslink density, the deformability of the polymer vesicles may be varied continuously from a fluid to nearly a solid particle.