Spontaneous formation of vesicles which occurs upon dilution of lecithin-bile salt mixed micellar solutions was studied using static (SLS) and dynamic (DLS) light scattering. Special attention was paid to the properties of the vesicles formed, and also to the mixed micellar precursors, since the structure of these mixed micelles is still controversial. We show how a self-consistent interpretation of the SLS and DLS data can be achieved using concepts from colloid and polymer physics and incorporating the effects of polydispersity and interparticle interactions. The micelle to vesicle transition is interpreted within the framework of current theoretical models for the spontaneous formation of vesicles.