We experimentally demonstrate transistor operation in a vertical p⁺-MoS₂/n-HfS₂ van der Waals (vdW) heterostructure configuration for the first time. The HfS₂/MoS₂ heterojunction transistor exhibits an ON/OFF ratio of 10⁴ and a maximum drain current of 20 nA. These values are comparable with the corresponding reported values for vdW heterojunction TFETs. Moreover, we study the effect of atmospheric exposure on the subthreshold slope (SS) of the HfS₂/MoS₂ transistor. Unpassivated and passivated devices are compared in terms of their SS values and I_DS-V_GS hysteresis. While the unpassivated HfS₂/MoS₂ heterojunction transistor exhibits a minimum SS value of 2000 mV/dec, the same device passivated with a 20-nm-thick HfO₂ film exhibits a significantly lower SS value of 700 mV/dec. HfO₂ passivation protects the device from contamination caused by atmospheric moisture and oxygen and also reduces the effect of surface traps. We believe that our findings will contribute to the practical realization of HfS₂-based vdW heterojunction TFETs.