The flag of the Vietnamese Constitutional Monarchist League (Vietnamese: Liên Minh Quân Chủ Lập Hiến Đa Nguyên Việt Nam), a Nguyễn Dynasty restorationist organisation based in the United States of America.
Apparently this version is a Vietnamese Monarchist flag with the name of the Vietnamese Constitutional Monarchist League added to it. 

http://www.oocities.org/vietmonarchy/ (Archive).
This flag is indeed used by Vietnamese Monarchists, at a website called "www.oocities.org/vietmonarchy", which describes itself with the intro "Vietnam, as any proud Vietnamese native can tell you, has a much longer and more glorious history than the three decades of war in the 20th Century which acquainted most people in the world with Southeast Asia. The Vietnamese have a history, stretching back for thousands of years, as an advanced, prosperous and highly civilized king-dom. Theirs was a nation of brave, innovative warriors, women generals, great scholars and revered poet kings." and is later attributed to have been written by a Christian Vietnamese Monarchist as it states: "This website is written by Joseph A. Crisp II and dedicated to Our Lady of Lavang. May our Holy Mother intercede for the people of Vietnam, be with them always and bring them peace. (The music playing is Dang Dan Cung, the imperial anthem of the Nguyen Dynasty.)" This indicates that my contact was correct that the flag is indeed "a generic Vietnamese monarchist flag" used to represent the Vietnamese monarchy. Upon further investigation Joseph A. Crisp II, a Texan monarchist (USA monarchist) is my (Donald Trung Quoc Don's) over a decade-long personal hero known as "The Mad-Monarchist" and knowing their investigative skills am inclined to believe the validity of their depiction of this flag, what is further interesting is the entry Further Reprehensible Conduct by the So-called "Vietnamese Constitutional Monarchist League" In this "final essay" by Joseph A. Crisp II notes that "Aside from calling me, and anyone who shows the slightest scepticism about their claims, a Communist the "Vietnamese Constitutional Monarchist League" under the leadership of one Nguyen Buu Chanh have now resorted to thievery in an effort to further their own aims. That's rights, this "prestigious" group which claims to have a small army of princes, aristocrats, military officers, doctors and professors, must it seems steal the work of this lowly Texan to spam the web with "press releases".". This indicates that the flag and likely many other Vietnamese monarchist symbols from this group were stolen and were likely not even created by them. Later Joseph A. Crisp II elaborates on this and claims that almost anything from the Vietnamese Constitutional Monarchist League was added from Joseph A. Crisp II's original website. "Further, using a myriad of aliases, they have stolen and reposted at another "prweb" site here almost every picture and illustration from my pages. You will notice they didn't even bother to change the custom backgrounds (flags, colors) or filenames before they re-posted their stolen material.". Later it was written that a lot about the Vietnamese Constitutional Monarchist League was added to the English-language Wikipedia was done so by its members as a PR stunt and that Wikipedia editors repeatedly removed their images and information because of their deceptive nature: "Apparently, this has become a pattern of behavior for this group. I have since become aware of their having stolen a number of my pages for posting at the "Wikipedia" on-line encyclopedia, with their own names &amp; opinions inserted and mine removed of course. This resulted in the editors of this website, as well as pushing unsubstantiated claims to take this action. They have also pasted copies of these hot texts and images on virtually every free forum they could find. It seems odd to say the least that a group which claims such exalted status would have to resort to plagarism, and from someone like me of all people, just to fill out the blanks in their "press releases".". This means that not only images but almost all content from the Vietnamese Constitutional Monarchist League was stolen from other Vietnamese monarchists. These claims regarding the plagiarism and false claims of legitimacy by the Vietnamese Constitutional Monarchist League apparently caused Joseph A. Crisp II to stop updating his website about the Vietnamese monarchy and Vietnamese monarchism because his website in particular was the target of this blatant copying by the group: "However, because their only source of historical writing and pictures seems to be stolen material from myself, allow me to repeat that I do not endorse them, support them or advise anyone else to have anything at all to do with them. They have proven their character and earned the reputation they have. However, for this same reason, I will not be adding any more essays or images from my collection to this website. Thank you, and remember, be vigilant." This might explain why the flag has mostly become associated with the Vietnamese Constitutional Monarchist League rather than other Vietnamese Monarchist groups because of this rampant plagiarism. So as far as I have been able to determine from various internet archives this flag is the original Vietnamese Monarchist flag and appropriated by the Vietnamese Constitutional Monarchist League who simply added their name to the flag.
Based on information from
Perspectives on World History and Current Events (PWHCE) Monarchy Links (Ecosia search / Source image).Source files
File:Flag of South Vietnam.svg
File:Vietnamese Dragon green.svg
File:Car flag of Bảo Đại, Emperor of Annam (1926-1945).svg
Donald Trung Quoc Don (徵國單), Sodacan, Goran tek-en and Great Brightstar.