In dilute aqueous solutions of ammonium perfluoropolyethercarboxylate (PFPE) and n-dodecylbetaine, for a small range of proportion of PFPE and betaine, we find evidence of the spontaneous formation of vesicles of a well-defined radius. This formation is tentatively explained by the fact that PFPE and betaine have only a partial miscibility allowing for a strong difference in composition of the outer and inner layer of the membrane of the vesicle. This can lead to a preferred radius of curvature for the membrane as described by Safran et al.