Overview
The Later Le presided over a Confucian state and a divided country, and both show in dress. Court costume was codified in fine detail on the Chinese model; meanwhile the lords governing the south used clothing to mark themselves off from the north, and the garment they promoted became, two centuries later, the national dress.
Key garments and style
Court and officials wore the cross-collar robe closing to the right, with cap and belt, colour and pattern assigned by rank under codes that were revised repeatedly. Confucian propriety pushed the local customs earlier dynasties had tolerated — blackened teeth, tattooing, bare feet at court — toward the margins.
The consequential act came from the south. In the 1740s Nguyễn Phúc Khoát required his subjects to wear trousers with a front-closing gown, distinguishing the southern domain from the Trịnh-controlled north where women wore the wrapped skirt and the four-panel áo tứ thân. The garment that resulted, the five-panel áo ngũ thân, is the direct ancestor of the áo dài.
So a dress decree issued to mark a political division became, once the Nguyen unified the country, the basis of a national costume.
Transition
The Nguyen dynasty extended that requirement over the whole country, and the five-panel gown spread north by law.