A 3D stackable and bidirectional Threshold Vacuum Switching (TVS) selector using the same WOx material as the RRAM element is reported. It provides the highest reported current density of >108 A/cm2 and the highest selectivity of >105. Stress test at high current density indicates >108 cycle capability for Reset/Set operation. A mechanism based on recombination of oxygen-ions and vacancies is proposed for the observed volatile switching of TVS. Utilizing the threshold characteristics of the TVS selector, a two-step reading waveform offers potential for 3D-stackable and 4F2 cross-point RRAM applications.