The so-called "vassal state" is a very complex historical concept. Historically, Japan was actually on the fuzzy line of the vassal of the Chinese Empire.
Relatively speaking, Japan's significance to the Chinese Empire is extremely limited. It is clear that the Chinese Empire never needed Japan as a vassal to buffer the powerful political forces from the eastern sea area. This fundamentally determines that the Chinese Empire's efforts to win over Japan are extremely limited, let alone the establishment of administrative management. . Therefore, Japan has long been isolated from the vassal structure of the Chinese Empire.
Of course, during Japan's long-term learning from China, there was a tributary relationship. Although the tributary state and the suzerain state are maintained through the tributary relationship, the existence of the tributary relationship does not necessarily mean the suzerain and the vassal. As early as the Han Dynasty, the countries of Japan, which had not yet been unified, sent envoys to the Central Plains Dynasty. During the period of Emperor Wu of the Han Dynasty, after the Han Empire destroyed Korea, the prefectures and counties were established, and the Japanese had more than 30 small countries communicating with the Han. All countries are called kingdoms, and they have been inherited from generation to generation.
All in all, in the history of Sino-Japanese relations, there have been times when a certain Japanese regime paid tribute to the Chinese Empire and was canonized, but on the whole, Japan has little strategic significance to the Chinese Empire (very lucky for Japan), saying that It is not appropriate for it to be a vassal state. Of course, this does not affect the objective fact that Japan has long been politically inferior to the Chinese Empire and has learned from China.