  A microscopic model which has proven useful in describing amphiphilic
aggregates as inhomogeneities of a fluid is extended here to study the case of
a two component surfactant mixture. We have chosen an effective interaction
between the amphiphiles that mimics the mixture of cationic--anionic
surfactants. In agreement with experiments, and other theoretical approaches,
we find regions where spherical vesicles are stable, with a well defined
radius. The experimental dependence of the radius on the chemical environment
is also qualitatively reflected in our model.
