With the advancement of additive manufacturing (AM) and the mass adoption of 3D printing technology, it is essential to shift focus to environmentally and economically sustainable materials. As the utilization of renewable feedstocks is quite limited in this context, the utilization of more bio-based raw materials in the ongoing development of AM represents an essential means of achieving this shift. In this work, vat photopolymerization 3D printing has been used to process vegetable oil-based (VO) resins with an ultralow concentration of 0.07 vol % nanocellulose fibrils (NFC) and crystals (NCC). The developed nanocellulose containing bio-based vat photopolymerization resin shows excellent shelf stability, enabling high-resolution printing. Compatibilization of the nanocellulose with the polymer matrix was achieved through the introduction of isocyanate or acrylate groups via reactions of acryloyl chloride (AC) and hexamethylene diisocyanate (HMDI) with cellulose surface hydroxyls. Surface functionalization results in ∼20–30% increases in interfacial adhesion and stress transfer, yielding significant improvements in mechanical performance (4× higher toughness, 2.4× higher tensile strength, and 2× higher tensile strain) in 3D-printed specimens. Fourier-transformation infrared (FTIR) spectroscopy complemented by solid-state nuclear magnetic resonance (NMR) techniques enabled a more detailed study of the chemical structure of these materials as well. Tensile performance comparison with literature data on VO-based natural fiber-reinforced resins showed that this work brought bio-based resins one step closer to competing with petroleum-based resins. The prepared VO/nanocellulose resins are promising candidates for high-performance bio-based resins derived from completely renewable feedstocks.