Okay, this is a really great question, and one that has a really interesting answer. The Tây Sơn rebellion/dynasty was led by three brothers from the countryside. This answer will require a lot of background information in order to answer in a way that will be clear, so I will not start with the rebellion.

Part I:

Instead, we need to go back further to the earlier part of the Lê Dynasty, which was the longest Vietnamese dynasty, lasting from 1428 to 1789. This was also the last dynasty of what could be called "Classical Việt Nam" (this period encompasses many of what are considered official "dynasties" but the really big important ones are the Ngô, Lý, Trần, and Lê). The Lê Dynasty can be split into three periods: the early Lê, the Mạc period (the Mạc are referred to as a dynasty in official records; they toppled the Lê for a period), and the Lê restoration. This last period started in civil war, as the restoration was a fight between the Lê/lords Trịnh and Nguyễn and the Mạc usurpers. This would last at least intermittently until 1677, when the Mạc were finally defeated for good.

During the period during and after this conflict, it became apparent that the families Nguyễn and Trịnh were the true power behind the Lê throne. Trịnh Kiểm took over the power of the Lê dynasty in 1546; this set the stage for a massive conflict. Nguyễn Hoàng, sensing danger, asked his sister, who was Trịnh Kiểm’s wife, to persuade her husband to send him away as a commander of the distant frontier area, Thuận Hoá in 1558. In this southern region, Nguyễn Hoàng continued paying taxes to the Lê-Trịnh in the north while building his political base. However, his son, Nguyễn Phúc Nguyên, stopped paying these taxes in 1602. This was the reason behind the Trịnh’s seven campaigns against the Nguyễn between 1627 and 1672, all of which were unsuccessful. For the next century, there was relative peace as the south began to forget the north.

During this entire period, a different kind of Vietnamese culture and society was developing around what is now central and southern Việt Nam. The Nguyễn controlled an area outside of the traditionally Vietnamese regions; Quảng Trị and Huế were areas that once were once under control of the Cham, an Austronesian-speaking group. Champa was an ancient kingdom that had long been at war with their Vietnamese neighbors to the north. The Vietnamese claimed this territory but never actually really controlled it. Over time the Vietnamese had expanded and had finally defeated Champa for good (after nearly being conquered by the Cham themselves). This expansion created a frontier where Confucian scholarship, though the state ideology under the Lê was more or less unknown and not thought of. Indeed, from the 17th century onwards, a new center of Vietnamese civilization was being firmly established: Huế was the political center that was established in addition to Thăng Long (old name of Hà Nội) and Thuận Quảng emerged as a powerful economic area of its own to challenge the economic supremacy of the Red River Delta in the north. This region had a different cultural background and the southern Vietnamese themselves recognized this, referring to their region as the "inner region" and the north as the "outer region". Two different ways of being Vietnamese were possible.

Continued...