Suspected proginetor of ao dai - Giao linh robe of the Le dynasty. Please check this thread for more information: http://www.eastbound88.com/showthrea...ctions-inside) 
Early Nguyen dynasty
To me it looked as though when buttons became popular, people just pulled the entire front flap over instead of leaving it cross-necked and secured by knot at armpit. The skirt was also replaced by trousers as decreed by Emperor Minh Mang.
The early ao dai was very loose, reminiscent of the giao linh in Le dynasty.
Just look at their chest area, you can feel that they were wearing several layers of robes inside, again it's reminiscent of how giao linh was supposed to be worn.
Progressively, you can see the sleeves are being tightened and people were wearing less layers of clothes.
The real change came with the tightening of the chest area and the modification of the collar
And since then it has looked pretty much like it does today