The invention relates to a process for coating a substrate (S) whereby a metal alloy layer comprising at least two metallic elements is continuously deposited on the substrate (S) by means of a vacuum deposition facility (1) comprising a vapor jet coater (7) for spraying the substrate (S) with a vapor containing the metallic elements in a constant and predetermined relative content, the vapor being sprayed at a sonic velocity. The process is more particularly intended for depositing Zn—Mg coatings.The invention also relates to a vacuum deposition facility (1) for continuously depositing coatings formed from metal alloys, for implementing the process.