Japan Airlines currently provides a number of flights from several origins, but like other international airlines in the areas, these flights are not direct and have to land in Japan before heading to Hanoi or Ho Chi Minh City. On the route from Japan to Hanoi, there are three daily flights offered by Japan Airlines, including two flights from Haneda - Narita and another from Kansai. The flight departs from Kansai international airport at 10.30, while flights from Narita often take off at 17.55 and 10.30 (local time).
Moreover, there is some additional flights on this route: one from Chubu departs at 11.15 on Tuesday, Thursday, Saturday and Sunday every week; and one from Fukuoka takes off at 10.30 on Tuesday and Saturday. The fare for one adult on the Economy class is approximately 1,411 USD for round trip.
From Japan to Ho Chi Minh City there are four daily flights: three from Narita and one from Kansai. The flights depart from Narita at 17.50, 10.00 and 19.00 and the one from Kansai takes off at 10.30.
There is also one additional flight from Fukuoka to Hanoi that departs at 10.30. The ticket price of those planes for an adult on the Economy class depends on how early you book your ticket.