Imperial Flag of Annam, raised over the Imperial Palace in Huế. Used by Emperor Bảo Đại (1926-1945), but maybe even during the whole Nguyễn dynasty (1802-1945).
Own work based on:✦ Nguyen Imperial Pennon (m1).png by x✦ Vietnamese Dragon green.svg by Goran_tek-en

Information from:

Hervé Calvarin, Olivier Corre: L'Indochine. Étude d'Histoire vexillologique. In: Franciae Vexilla. Numéro Spécial 2005, ISSN 1270-0096, S. 89-126; 147-162.
Requested by and knowledge from Antemister
Goran tek-en