Bart, your coin appears to be a Chinese original, or direct copy. Japanese specimens usually show slight magnetism. They were made during the Chinese reign as inscribed in great quantity for export as well as domestic use. It is just those widely exported cash that were often copied, as these were in Vietnam and Indonesia also. The originals exhibit only miniscule variations. They were the predominant cash coin in Japan for some 100 years.
There are several Vietnamese reign titles with this same top character, but none with both the same. "Vinh huu" 1735-40 is a different bottom character. No coins were issued with that inscription officially, nor to my knowledge, privately. "Vinh lac" is the Vietnamese pronunciation of the Yung le characters. Numismatically that usually refers to pieces in zinc made with that inscription by the Nguyen lords in S. Vietnam. Along with nearly a hundred other inscriptions. There are other Vietnamese copies, some with their own distinct style of calligraphy, some intended to pass as originals. Much cruder copies made in Java are known as "Shima-sen" (Japanese = island cash).
Figleaf, if your copy (a beauty) is magnetic, it's Japanese. All of those have been cataloged, so you can probably track it down.