Vesicles in a state of mechanical equilibrium (but not necessarily in a state of diffusional equilibrium) are considered. The packing conditions are derived for vesicles on the basis of considering a vesicle as a whole and of a more detailed analysis of both the monolayers of the vesicle. The packing conditions prove to be different for the outer and the inner monolayers of the vesicle. The generalized Gibbs—Curie principle is derived for a vesicle. Some difficulties which arise are related to the fact that, in contrast to the case of micelles and small crystals, the vesicle in a state of difrusional non-equilibrium should be treated as a three-phase system with two interfaces. The generalized Gibbs—Curie principle is applied to the analysis of the vesicle—disc-like micelle phase transition. Conditions when the formation of vesicles becomes energetically more favorable than that of disc-like micelles are shown to be easily realizable. Under such conditions, vesicles form spontaneously from bilayers without ultrasonic treatment.