File:Sun emblem of the Nguyễn Dynasty.png

English: A depiction of the sun that was used by the Dynasties of Imperial China, Royal Korea, and Imperial Vietnam as a symbol of the Dynasty (with a moon being the analogous symbol of the State). This specific image of the Nguyễn Dynasty period sun ☀ was extracted from the coat of arms of Hanoi under the French protectorate of Tonkin (the Northern half of the Nguyễn Empire under French domination).

This symbol was used throughout Chinese cultural countries as a symbol of the reigning Dynasty and was often accompanied by a pair of Dragons in the clouds.

From what I can tell the sun emblem from the Hanoi coat of arms is a French interpretation of the symbols of the Nguyễn Dynasty, actual Chinese-style suns are often depicted differently. It is also important to note that Chinese symbolism is vastly different from European symbolism, this I attribute to likely be a result of the paraphernalia, most notably cash coins and Chinese-style seals, the designs of these things are largely just inscriptions and the wide varieties of Chinese calligraphy show their differences.

Meanwhile Lydian-style coins have always contain a lot of symbols, usually a portrait of a reigning monarch on one side and an important symbol on the other, meanwhile seals from "West of the Himalayas" also contain the symbols of authority. Because of this different coats of arms were created to differentiate the many different peoples and countries, tribes and organisations. In the Chinese cultural countries this was different, only Japan has had a strong and old heraldric tradition, the other Chinese cultural countries tended to use the same symbols over centuries if not longer than a millennium. Before the establishment of the French protectorates of Annam & Tonkin the Nguyễn Dynasty didn't have "a coat of arms" as any European country had, but instead different levels of authority were represented using different symbols such as Dragons, Chinese Guardian Lions, "Phoenixes", Etc. as well as office seals. So this emblem would very much be out of place as "a coat of arms of the Nguyễn Dynasty" (something that was only adopted after the French domination began). While both "Dynastic" and "State" symbols existed, these were the same in all Chinese cultural countries and no concept of "national symbols" like in Europe existed, the flags from this period were often imperial standards and not "national flags" as we know them today.

Another important difference is that the sun wasn't consistently depicted in the same style but many different versions existed almost every time it was depicted. The best indicator of national symbols would be on coins, other than cash coins the coins of the Nguyễn Dynasty, Manchu Qing Dynasty, Joseon, and the Japanese Empire all used Dragons, thus the "national symbol" of all Chinese cultural countries during this time would have been the Chinese dragon using this logic. Also notice that this sun emblem is usually accompanied by a pair of dragons.