Computational analysis was used to determine whether there were lateral domains of different thickness forming in the membrane of different vesicle compositions (see SI Section 4). The analysis compares the apparent membrane thickness along a section of a hybrid membrane with the bilayer thickness measurements of a pure lipid and pure polymer vesicle and computes the likelihood that a measurement in the hybrid membrane falls within the thickness distribution attributed to the pure lipid or pure polymer vesicle. The hybrid thickness measurements are sorted into thick, polymersome-like (> 70 Å) and thin (< 60 Å) membrane thickness categories. . Blue indicates 0% likelihood that membrane is in polymersome-like phase, while red indicates 100% likelihood that membrane is in polymersome-like phase.