Most cultures have a history of modest dressing, its only recently that people started wearing exposing clothes that are tight and see through. The film Heave & Earth contains many images of modest dressing, I'm not sure if they are indeed traditional Vietnamese outfits but they are beautiful none the less. The characters in Good Morning Vietnam also wear similar clothes.
This one on the left is quite tight, a good reminder that the headscarf should be drapped over the bosom.
Here's two similar abayas from Aab