We report a small-angle neutron-scattering (SANS) study of the structural properties of mixed bile salt-lecithin micelles and vesicles. The SANS data from a mixed micellar sample are in agreement with the presence of flexible cylindrical micelles. We present an expression for the scattering cross section of polydisperse wormlike micelles which is capable of quantitatively describing the scattering intensity I(q) over an extended range of scattering vectors and which allows us to incorporate overall size, polydispersity, flexibility, and local structure of the micelles in a self-consistent way. At concentrations below the micellar phase limit, where a spontaneous micelle-to-vesicle transition can be observed, we find that the scattering intensity at higher values of q is not consistent with a population of vesicles only and that a coexistence of vesicles and wormlike micelles occurs.