The morphological stability of vesicles consisting of an amphiphilic poly(methacrylic acid)-block-poly(methyl methacrylate-random-methacrylic acid) diblock copolymer, PMAA-b-P(MMA-r-MAA), was investigated against the external stresses of pH, salt concentration and polyamine. The worm-like vesicles underwent a partial fusion at pH 12, however, they retained the worm-like shape at pH 13 due to electrostatic repulsion. On the other hand, the spherical vesicles were completely fused at pH 12, transformed into a sheet and did not retain their shape under the higher basic condition. Similarly, the worm-like vesicles retained their morphology in 0.1-mol% solutions of sodium chloride and sodium dodecyl sulfate, while the spherical vesicles caused division and fusion even at much lower concentrations. Poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) transformed the worm-like vesicle into a cleavable sheet, while it changed the spherical vesicles into a sheet without a specific form. It was found that this transformation based on the acid-base interaction between the carboxylic acid of the MAA block and the amine of the PDMAEMA was dependent on the molecular weight of the PDMAEMA. The short PDMAMA retarded the fusion of the vesicles.