Flag that is used by various medias to represent the Vietnamese Nguyễn dynasty under Gia Long.
References:
Làng Nam
Phố núi và bạn bè
FOTW
Markodehaeck-Usages in real life:

Reddit-Flag of Đại Nam at tomb of Gia Long
Vietnamnet-Gia Long shrine, the resting place of the Nguyen Dynasty’s first king (16th photo)
Havsjö