Nanoparticle occlusion within growing crystals is of considerable interest because (i) it can enhance our understanding of biomineralization and (ii) it offers a straightforward route for the preparation of novel nanocomposites. However, robust design rules for efficient occlusion remain elusive. Herein, we report the rational synthesis of a series of silica-loaded poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate)-poly(ethylene glycol dimethacrylate)-poly(methacrylic acid) tetrablock copolymer vesicles using polymerization-induced self-assembly. The overall vesicle dimensions remain essentially constant for this series; hence systematic variation of the mean degree of polymerization (DP) of the anionic poly(methacrylic acid) steric stabilizer chains provides an unprecedented opportunity to investigate the design rules for efficient nanoparticle occlusion within host inorganic crystals such as calcite. Indeed, the stabilizer DP plays a decisive role in dictating both the extent of occlusion and the calcite crystal morphology: sufficiently long stabilizer chains are required to achieve extents of vesicle occlusion of up to 41 vol %, but overly long stabilizer chains merely lead to significant changes in the crystal morphology, rather than promoting further occlusion. Furthermore, steric stabilizer chains comprising anionic carboxylate groups lead to superior occlusion performance compared to those composed of phosphate, sulfate, or sulfonate groups. Moreover, occluded vesicles are subjected to substantial deformation forces, as shown by the significant change in shape after their occlusion. It is also demonstrated that such vesicles can act as “Trojan horses”, enabling the occlusion of non-functional silica nanoparticles within calcite. In summary, this study provides important new physical insights regarding the efficient incorporation of guest nanoparticles within host inorganic crystals.