In two-dimensional transition-metal dichalcogenides (TMDCs), the local electrostatic field of the chalcogen p -orbitals dictates the crystal field splitting of the transition metal d -orbitals, stabilizing diverse electronic states from semiconducting and semimetallic to exotic topological phases. On-demand substitution of the lattice chalcogens is therefore an efficient route to tuning TMDC electronic phases. Here, we custom-molded van der Waals (vdW) heterostructures by layer-by-layer vapor-phase chalcogen substitution starting from p -type WSe 2 multilayers into which we locally inscribed insulating WO x , n -type WS 2 , and semimetallic WTe 2 . This spatially precise conversion seamlessly integrated distinct electronic phases, both vertically and laterally, into modular vdW semiconductor heterostructures that are otherwise inaccessible. As a demonstration, we integrated WO x /WSe 2 local gate stacks to build ultrascaled metal oxide–semiconductor field effect transistors (MOSFETs), achieving an unprecedented subthreshold swing at subnanometer equivalent oxide thickness.