Micrometer-sized spheres have been found to assemble from homopolymer electrolytes and small, multivalent counterions in water. In contrast to previous efforts, these vesicles do not use preformed templates, do not require block copolymers, and do not necessarily employ nanoparticles. We have investigated the requirements for vesicle formation with regards to both components of the assembly. Self-assembly occurs with a variety of poly-amino acids and counterions, all of which require a minimum number of charged groups to promote non-covalent crosslinking. We show how the assembly process is controlled by pH and how, in consequence, the pKa's of the reactants can be used to reliably predict sphere formation. By varying the nature of the small counterions, we have determined the requirements for assemblies. The assemblies have been further investigated using confocal microscopy and fluorescent labeling of the different components.