Presented are electronic power module assemblies with direct-cooling vapor chamber systems, methods for making/using such power module assemblies, and vehicles equipped with such power module assemblies. A power module assembly includes an outer housing with an internal coolant chamber that circulates therethrough a coolant fluid. A power semiconductor switching device is mounted to the module's housing, separated from the coolant chamber and isolated from the coolant fluid. The power device selectively modifies electric current transmitted between a power source and an electrical load. A two-phase, heat-spreading vapor chamber device includes an outer casing with a casing segment that is mounted to the module housing, fluidly sealed to the internal coolant chamber and exposed to the coolant fluid. Another casing segment includes an inboard-facing casing surface that is mounted to an outboard-facing surface of the power device, and an outboard-facing casing surface mounted to an inboard-facing surface of the power device.