Vat photopolymerization is an additive manufacturing technique that utilizes photosensitive resins to fabricate 3D polymeric objects with high precision. However, these objects often lack mechanical strength. This study investigated the strengthening of a resin based on epoxidized soybean oil acrylate, specifically designed for vat photopolymerization, by the in situ formation of hydroxyapatite nanocrystals. First, a stable alpha tricalcium phosphate (α-TCP)-resin feedstock mixture was developed (~30 vol.% α-TCP), which proved suitable for fabricating monoliths as well as complex triply periodic minimal surface (gyroid, diamond, and Schwarz) porous structures through vat photopolymerization. The results demonstrated that the incorporation of α-TCP particles led to a significant mechanical improvement of the resin. Second, post-printing hydrothermal treatments were utilized to transform the α-TCP particles into hydroxyapatite crystals within the resin. It was observed that the space between hydroxyapatite crystals within the composites was occupied by the cured resin, resulting in a more compact, stronger, and mechanically more reliable material than the porous hydroxyapatite produced by the hydrolysis of α-TCP mixed with water. Moreover, water absorption during the hydrothermal treatments caused the plasticization of the cured resin. As a consequence, the hydroxyapatite-resin composites displayed slightly lower mechanical properties compared to the as-printed α-TCP-resin composite.