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.