Originally Posted by
Tokyoite
Tokyo is simply a very very long stop over on your route from country A to country B.
So, you must return to a country different from where you departed from, for it to qualify as a "one way award".
HKG/Taiwan and China are treated as separate countries.
This is the crux of it. It's not just some AS award routing rule; it'd be illegal for AS/JL to sell you a single ticket for what is, in the end, a domestic Vietnamese trip.