SPA of 50 mol% PBd22-PEO14 hybrid vesicles confirms a homogeneous distribution within a single vesicle. Previous literature has shown good mixing between these lipid and polymer components in hybrid membranes with a monomodal distribution of lipids and polymers between different vesicles centred on the mean composition. The cryo-EM and cryo-ET data also shows no evidence of phase separation into polymer-rich and lipid-rich domains within individual vesicles of hybrid samples. Together, this strongly implies a bistability between two membrane structures that are close in free energy: thin membranes with a structure dictated by the preferred bilayer ordering of the lipids and thicker membranes with the membrane structure dictated by the partially interdigitated bilayer structure of the block copolymers. Intermediate membrane structures do not appear to be energetically favourable for these hybrid lipid-block copolymer blends. We also find that it is energetically unfavourable for domains of thin and thick membrane structures to coexist in the same vesicle, likely due to the high line tension of domain boundaries. As such, each vesicle selects between the lipid-like thin membrane structure or the polymer-like thick membrane structure during formation. This selection may be partially driven by differences in the relative vesicle compositions across the width of the monomodal mixing distribution of these components that has been previously reported. These novel findings on the membrane structure of hybrid POPC/ PBd22-PEO14 vesicles have implications for their technological applications. For example, it is interesting to note that previous work inserting cytochrome bo3 into these vesicle compositions finds that optimal stability of this enzyme's function is within 50 mol% PBd22-PEO14 hybrid vesicles. Two distinct membrane structures exist at this composition: future work could investigate the distribution of these membrane proteins between thick and thin hybrid vesicle membrane populations and how each of these contribute to enzymatic function and stability. Different coexisting membrane populations will also have implications for the use of hybrid vesicles in drug delivery, where membrane thickness would influence the release kinetics of bioactive compounds. A distribution of release rates within a formulation might be beneficial for drug pharmacokinetics in some long-acting delivery applications.