I think this is a funny case of a false cognate. Vapeur is the French word for steam. Vapeur sounds like vapor, and steam is a vapor. So a steam gun is still so

I think this is a funny case of a false cognate. Vapeur is the French word for steam. Vapeur sounds like vapor, and steam is a vapor. So a steam gun is still sort of a vapor gun, but it's definitely a vapeur gun.