We demonstrate a microfluidic approach to the production of block copolymer vesicles via flow directed self assembly in a segmented gas-liquid device. Chemical conditions that favor spherical micelles in the bulk are found to yield a nearly pure population of vesicles on a chip-a transformation of two full morphological steps-because of a coalescence mechanism enabled by high shear. The production of polymeric vesicles via top-down control in a microfluidic device enables new processing routes to applications including drug delivery formulations in the lab-on-a-chip.