The presence of thin and thick membrane populations in hybrid lipid-block copolymer vesicle samples might be interpreted as poor mixing between these components and a strong segregation into different vesicles. However, previous studies utilising POPC and PBd22-PEO14 have reported good mixing between these components into hybrid vesicles with a monomodal distribution of lipids and polymers centred around the sample mean. Giant unilamellar vesicles (GUVs), created by the electroformation method, have been studied containing a labelled polymer and lipid-like fluorescent probe, where these components were well mixed across 25 mol%, 50 mol% and 75 mol% components with a single broad distribution of relative fluorescence intensities across the GUV population. Similarly, Lim et al. used flow cytometry of LUVs containing mixtures of POPC and PBd22-PEO14 containing fluorescent lipid and polymer probes to show good mixing between these components in individual large unilamellar vesicles (formed by thin film rehydration and extrusion) in their samples. Assuming good mixing of lipid and polymer components between individual vesicles is given (with a monomodal relative distribution of components centred on the mean sample composition), a bistability in membrane structure between the thin and thick configurations is implied, dictated by the preferred structure of the lipids or the polymers. It can be inferred that membranes of intermediate structure between these two states are not energetically favourable and hence the vesicle selects a membrane structure in one of these two states upon formation. The homogeneity of the membrane structures in individual vesicles also implies that coexistence of these two membrane configurations in a single vesicle is also not energetically favoured, likely due to the line tension that would arise between these domains. This observation has interesting implications for the application of hybrid vesicles in biotechnology and nanomedicine, where two distinct membrane structures can coexist, which could give rise to differential vesicle properties in a single sample.