Fe-based potassium Prussian blue analogues (K-PBAs) are traditionally used as K-ion battery cathodes. Interestingly, K-PBAs are appealing cathodes for Na-ion batteries (NIBs), due to the increased cation intercalation voltage compared to Na-PBAs. In such a hybrid NIB cell, where Na+ is in the electrolyte and K+ is in the PBA cathode, cation intercalation and electrochemical performance of the cathode can be significantly affected by [Fe(CN)6]4- anion vacancy. This work studies the effect of [Fe(CN)6]4- anion vacancy in K-PBAs on regulating K+/Na+ intercalation mechanism in hybrid NIB cells, by comparing two K-PBA cathodes with different vacancy contents. Experimental and computational results demonstrate that introducing a level of anion vacancy can maximize the number of K+ intercalation sites and enhance K+ diffusion in the PBA framework. This facilitates K+ intercalation and suppresses Na+ intercalation, resulting in a K+-dominated and high-discharge-voltage ion storage process in the hybrid NIB cell. The K-PBA cathode with 20% anion vacancy delivers 127 mAh g-1 at 50 mA g-1 and 63 mAh g-1 at 500 mA g-1, as well as retains 87% and 77% capacity after 100 and 300 cycles, respectively. It completely outperforms the counterpart with 7% anion vacancy, which exhibits increased Na+ intercalation but overall deteriorated ion storage. Our results show the promise of hybrid battery systems and the crucial role of vacancy regulation in designing electrode materials for these systems.