A novel methodology harnessing the synergistic influence of bimetallic and non-metallic dual vacancies within a unified catalyst for enabling highly efficient bifunctional photocatalysis encompassing oxidation and reduction processes is presented. ZnIn₂S₄, engineered to possess concurrent zinc and sulfur dual vacancies (ZnIn₂S₄-V_Zn+S), underwent synthesis and rigorous characterization employing atomic-resolution HAADF-STEM. This tailored catalyst was subsequently employed for pivotal photocatalytic processes, including nitrogen reduction (pNRR) and the photooxidative degradation of hexachlorobenzene (HCB). Computational analyses using Density Functional Theory (DFT) unveiled site-specific activation facilitated by Zn and S dual vacancies, activating water molecules and nitrogen, culminating in a synergistic effect driving ammonia synthesis. Additionally, X-ray Absorption Near Edge Structure (XANES) spectroscopy elucidated the role of photogenerated electrons confined within the sulfur vacancy, utilizing In³⁺ as an intermediary for electron migration, instigating a reaction with N₂ to yield NH₃ (In³⁺–N₂ + H⁺ + V_S(e⁻) → In²⁺ + NH₃), further augmenting the collaborative effect of dual vacancies on nitrogen reduction. Furthermore, the Zn and S vacancies emerged as active sites for hydroxyl and superoxide radical generation, facilitating enhanced participation of photogenerated carriers in radical generation reactions. This distinctive electron aggregation pathway engendered significant synergy, markedly enhancing the photodegradation prowess. Thus, the observed synergistic effect of site activation between Zn and S vacancies yielded a cumulative effect surpassing individual contributions (1 + 1 ≫ 2), thereby facilitating efficient photoreduction and photooxidation.