This does not strike me as a traditionally made knife from SE Asia. Is it a slip joint mechanism? If so, then it is not likely native to the region. Could be local scales on a knife originating elsewhere, or a knife "obtained" from a foreigner.
On looking closer at your pictures, there appears to be a clasp lock with a ring attached to release the lock. This type of securing device does appear on French and Chinese folding knives, and therefore could be expected in a Viet knife of that period (although I don't know that for sure).
I'm very wary of stories about knives taken from an enemy combatant.
Ian.