There exists a large demand for the fabrication of polymeric engineering parts by additive manufacturing. While many available thermoset photolithography resins achieve desirable performance capabilities, these systems commonly exhibit low fracture toughness due to their highly crosslinked structures. This limits the application of additively manufacture thermosets. A well-established method for toughening conventional thermosets is by secondary phase modification via functionally terminated butadiene-acrylonitrile (TBN) elastomers. However, due to poor compatibility with acrylate-based resins and high viscosity, these additives have yet to be implemented within photolithography resins. In this study, a low viscosity, bio-based, liquid rubber (Oct3) is added to photocurable vinyl ester resins and its toughening effect on printed thermosets was observed. The performance of printed thermosets modified with Oct3 is directly compared to thermosets containing vinyl-terminated butadiene acrylonitrile. Processable conditions were maintained when Oct3 was added to the base resin in various amounts (5-15 phr). Fracture toughness (KQ) of modified thermosets was observed to increase from 0.48 ± 0.01 MPa∙m1/2 to 0.83 ± 0.14 MPa∙m1/2 when 15 phr Oct3 was incorporated to the resin.