The flag employs colors of Five Elements and the Han character is used for the name Tran.
Photo of the modern use of the flag can be seen here. Consequently, it must have been used by the armed forces of South Vietnam, but currently I know no other details about its use, especially the circumstances of its adoption. Tomislav Todorovic, 19 February 2016
Saint Tran is Tran Hung Dao, Vietnamese prince and general noted for his victories over Mongols.1 He is considered a national hero in Vietnam, and worship of him is part of the Vietnamese folk religion.2 Tomislav Todorovic, 19 February 2016 Sources: 1 – Wikipedia article 2 – Wikipedia article