Electrostatic interactions play a prominent role in setting the phase behavior of mixtures of oppositely charged surfactants and lead to the formation of a variety of microstructures, including a novel equilibrium vesicle phase. Addition of a monovalent salt alters the electrostatic contribution to the free energy of aggregation and thus changes the equilibrium phase behavior and aggregate properties. Here we present observations for the catanionic mixture cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) with added sodium bromide (NaBr). The phase behavior of this surfactant system changes markedly when electrolyte is added. At certain compositions, there is a vesicle-to-micelle transition with increasing salt concentration, and surface charge density measurements, deduced from electrophoretic light scattering experiments, show that aggregate composition changes with added electrolyte. A thermodynamic cell model has been developed, supported by a new method of calculating vesicle electrostatic free energy, to quantify the various free energy contributions to aggregation. The cell model provides insight into how solution conditions such as composition and ionic strength affect phase behavior and related properties.