Ironically, the post-colonial borders
in mainland Southeast Asia look remarkably close to the immediate pre-colonial borders.
The European drawing of odd, straight lines separating tribes was much less pronounced here than in Africa.
To the extent many tribes were separated, it was by local kingdoms in the region, not the Europeans.
Now one thing that the European presence probably did do was slow down the rate of change or borders. For instance, Laos and Cambodia, while they existed as native kingdoms, probably would have been completely partitioned between Siam and Vietnam without French intervention.
The Sino-Vietnamese border probably would not have changed at all, its been remarkably stable for 600 or so years. Interesting book on this by David C. Kang that I recommend, "East Asia Before the West: Five Centuries of Trade and Tribute"
insular Southeast Asia is a whole other ball of wax of course.