I am going to make a guess with the hypothesis that teams stayed in Japan and visited somewhere (Mt. Fuji, Kyoto, Nagoya or Sendai are the top 4 on my list. I've been to all except Mt. Fuji (sort of visited in AR9) and they are all worthy of an AR15 visit.
From Kyoto or Nagoya, teams would go the short distance to the Osaka airport. There is an airport at Sendai, but it is possible to get to Narita airport so either one could have been used.
The Sendai airport flies only to Dalian and Beijing on Tuesdays and only twice on a Tuesday morning to Narita, but there are freqeuent flights to Osaka. So it looks like the Osaka to HoChiMinh City would be likely either way. I found only one flight combination that made sense:
AN 6064 operated 0810 to Okinawa
Change Okinawa to Taipei to CA121
Change Taipei to CA783 arrives SGN 1620
I am not saying it did happen, but it is logical and could have happened. Another possibility is that teams went to Okinawa or Taipei from Tokyo, did a leg and then sent on to Ho CHi Minh City. Nonstops from Taipei to SGN are CA 781 0730 0950, Eva 391 0920 1140 and CA783 1355 1620.