The Vietnamese have been advancing southward for centuries.If SEA were to be redivided to return to "historical" boundaries, the first problem would, of course, be which year or era it would be pegged at to say "this is the proper configuration." If you go back far enough Viet Nam becomes limited to areas around Ha Noi. Or perhaps that was "properly" part of China. Is the US to be redefined as the original thirteen states plus, perhaps, the Ohio Territory? or do we go back to 1600 for our definition of proper boundaries?
Claiming a territory because people of the same ethnicity live there is tribal rather than national. Besides, mixed with those Khmer Krom in the southern tip of Viet Nam is a much larger number of Kinh- ethnic Vietnamese.
And that work "Khmer" is pronounced more like "khmai" by such Cambodians as I know.