Self-assembled polymeric aggregates are generally polydisperse in morphology due to the existence of many metastable states in the system. This shortcoming becomes a bottleneck for preparing high quality self-assembled polymeric materials. An important concern is the possibility of controlling morphological polydispersity through the modulation of the metastable states. In this study, both simulative and experimental results show that the metastable states can be modulated. As a typical example, the morphological polydispersity of amphiphilic ABA triblock copolymer vesicles have been successfully controlled by shear flow. A higher shear rate results in more uniform and smaller vesicles. However, if the shear rate is extremely high, small spheres and short rods can be observed. These findings not only give a deeper insight into the metastable behavior of self-assembled polymeric aggregates but also provide a new strategy for improving the uniformity of vesicles.