Three-dimensional porous materials with bicontinuous open porosity have emerged as a novel and versatile class of functional materials with applications spanning various fields. Among the various methods for fabricating such materials, top-down dealloying stands out as one of the most effective approaches. Vapor phase dealloying (VPD), capitalizing on differences in saturated vapor pressure between alloy constituents to selectively remove less stable elements or phases, offers great promise for creating porous structures, encompassing both active metals and inorganic elements. When conducted at elevated homologous temperatures, VPD results in much richer phase transformations compared to conventional electrochemical dealloying, intimately linked with the development of porosities. Here, we investigate the evolution of porosity and phases during dynamic vapor phase dealloying, employing a nickel-magnesium (Ni-Mg) alloy as a model system. We observe that the VPD of Mg2Ni proceeds through stepwise phase transformations (Mg2Ni ➔ MgNi2 ➔ Ni) with the formation of bicontinuous porosity in the first step. Both phase transformations occur in a layer-by-layer fashion, with kinetics primarily governed by Knudsen diffusion of Mg vapor within the developed pore channels. Our in-situ transmission electron microscopy (TEM) investigations unveil that synergetic surface and volume diffusions drive concurrent evolutions of porosity and phases. Additionally, thermodynamic analyses reveal that the distinct sublimation enthalpies of various Ni-Mg intermetallics endorse sequential, temperature-dependent phase development in the entropy-controlled VPD process. We propose the utility of the solid-gas phase diagram at low pressure as a practical guide for designing VPD experiments to achieve selective phase control in nanoporous binary alloys. This study provides essential insights into the thermodynamics and kinetics underlying VPD, offering a robust foundation for the creation of functional nanoporous materials.