Enzyme-loaded synthetic vesicles have attracted great attention for their feasibility to exert the efficient and prolonged functionality of loaded enzymes in harsh environments, such as in vivo. However, several issues remain regarding the optimization of their structures toward practical application. Herein, we fabricated polyion complex vesicles (PICsomes) loaded with l-asparaginase (ASNase@PICsomes) and conducted a detailed characterization to ensure their utility as nanoreactors functioning under the harsh in vivo environment of the bloodstream. ASNase@PICsomes showed 100 nm-sized monodispersed vesicular structures. Fluorescence cross-correlation spectroscopy revealed essentially no empty PICsome fraction in the product, indicating the quantitative formation of ASNase@PICsomes. Furthermore, fluorescence anisotropy measurement showed that the loaded enzymes were located essentially in the inner aqueous phase of PICsomes, being successfully segregated from the external environment. ASNase@PICsomes exhibited significantly prolonged enzymatic reaction compared with free ASNase after systemic injection into mice, corroborating their functionality as in vivo nanoreactors working under the blood circulation.