Overview
The main written source is a hostile one: Chinese accounts of embassies to Đại Việt, which record what struck a visitor as strange — bare feet at court, short hair, tattoos on men of rank, teeth lacquered black. The garment itself is the áo giao lĩnh, a robe crossing left over right, worn long by officials and short by everyone else, over loose trousers. Colour and fabric marked rank; cut, largely, did not, and that flatness is what the envoys noticed.
How it is worn
The robe wraps and ties, with a sash at the waist, and the sleeves are wide on court dress and narrow on working dress. Tattooing is the part that needs explaining: the Trần kept a court practice of tattooing dragons on the thigh, and soldiers of the royal guard bore characters on their chests, so the marks were a uniform that could not be taken off. Bare feet in the palace were normal enough that a Chinese envoy recorded it twice, and the practical dress of the delta — layers that dry, trousers that roll — sits underneath the whole thing.
History
This is the wardrobe of the dynasty that beat three Mongol invasions, and the tattooing is usually reported alongside that fact. It ended with the dynasty: the Hồ and then the Ming occupation pressed Chinese dress and hairstyle on the court, and the Lê restoration afterwards rebuilt a court costume that was more Chinese in its regulations than the Trần had ever been. Barefoot officials and tattooed guardsmen do not reappear.