Interesting anecdote on the double swords. The idea they may be presents for certain occasions would help explain why there are so many of them, often with blades that wouldn't be serviceable. There is also a belief in some parts of China that hanging a bow in the house increases the chance of a boy. That part seems to be true, I have about 20 composite bows in the house and we are indeed expecting a boy.
As for this knife, I agree it's probably Vietnamese.
Not only the plum decor on the handle is very Vietnamese, but also the guard with grooved edges is a feature more often encountered on Vietnamese arms.
See "L'Art a Hue" for an excellent overview of typical Vietnamese decorative motifs. Many are strongly inspired by Chinese designs, but all have this Vietnamese twist as observed on the decor of this knife.
A digital version of the book can be found here:
https://archive.org/details/larthunouvelle00asso