The basic dress/coat/top, except that diagonal closure should be higher, ending just beneath the armpit. It's fastened maybe thrice up to that point, then open all the rest of the way (but with closures down the whole length), exposing torso.
The traditional áo dài is Vietnam's national, two-piece dress. It is fitted from the waist-up and the rest of the dress is loose. On formal occasions, matching circular headgear, khăn đóng, is also worn.