Thermoplasmonic nanoparticles, known for releasing heat upon illumination, find diverse applications in catalysis, optics, and biomedicine. Incorporating plasmonic metals within organic vesicle membranes can lead to the formation of nanoreactors capable of regulating temperature-sensitive microscopic processes. Yet, the controlled formation of stable hybrid vesicles displaying significant thermoplasmonic properties remains challenging. This work presents the in situ synthesis of highly efficient thermoplasmonic polymer vesicles, or hybrid polymersomes, by nucleating ∼2 nm gold nanoparticles within preformed polymersome membranes. This process preserves the vesicles' morphology, stability, and overall functionality. Despite the small size of the embedded plasmonic nanoparticles, these hybrid polymersomes can efficiently convert laser light into a notable temperature increase on a larger scale through collective heating. We develop a theoretical framework that rationalizes the structure-property relations of hybrid polymersomes and accurately predicts their collective thermoplasmonic response. Finally, we demonstrate the biomedical potential of our polymersomes by employing their photothermal properties to induce the hyperthermal death of cancer cells in vitro, an effect amplified by their superior cellular uptake. We envision that these hybrid polymersomes will evolve into a versatile platform for precise control over nanoscale chemical and biological processes through plasmonic heating, unlocking numerous opportunities across various scientific and medical contexts.