In this study, we report the preparation of branched block copolymer assemblies by photoinitiated reversible addition-fragmentation chain transfer (RAFT) self-condensing vinyl dispersion polymerization in water at room temperature mediated by a binary mixture of a macromolecular RAFT (macro-RAFT) agent and chain-transfer monomer (CTM). Decreasing the [macro-RAFT]/[CTM] ratio led to the formation of branched block copolymers with higher molecular weights and higher degrees of branching. Moreover, branched block copolymer assemblies with various morphologies (including worms and vesicles) were prepared by changing the [macro-RAFT]/[CTM] ratio and a morphological phase diagram was constructed. The obtained branched block copolymer vesicles can be used as seeds for further chain extension via seeded polymerization of a water-miscible monomer or a water-immiscible monomer. Finally, reduction-responsive branched block copolymer assemblies were prepared using a disulfide-linked CTM, demonstrating the potential of this method for the preparation of stimuli-responsive polymers.