A method deposits a metal alloy coating on a substrate using a vacuum deposition facility. The facility is equipped with a vapour generator/mixer comprising a vacuum chamber enclosure provided with an inlet and an outlet for the substrate. The enclosure includes a vapour deposition head and an ejector is provided to create a jet of metal alloy vapour of sonic velocity towards the surface of the substrate and perpendicular thereto. The ejector is in sealed communication with a separate mixer device, which is itself connected upstream to at least two crucibles respectively, these containing different metals in liquid form, each crucible being connected to the mixer by its own pipe. The method uses a series of partitions to create alternating layers of metal vapours. The metal vapours enter the mixer inlet of the mixer at a velocity from 5 to 50 m/s to provide better homogeneity.