hm, well heres what i know of the Dai Viet:
1) They were more organized and disciplined than the Champa, however for a long time they were not able to defeat them
2) They fought off constant invasions from the Mongols and the Chinese and raids from the Champa
3) Later on they recieved weapons and support from European powers such as the Dutch and Portugese, and their astonishingly quick adaptation of firearms allowed them to eventually defeat the Champa and make them subserviant (much as the Siamese were able to do with the Khmer)
4) They, like the Champa, relied on light infantry and skirmishing tactics, also often fighting where the enemy couldnt fight back with their full potential/strength
hope it helps.....and i could probably provide a unit roster too, should u need it