I think it will be determined by your first flight as your journey ends in the U.S., and IATA rules state that in case the itinerary starts or ends in the U.S., the carrier of your first flight determines your luggage allowance.
Your first carrier however may decide to delegate luggage allowances to the MSC rules, in which case you would have the allowance of the 2nd leg (HND-SFO) as that flight crosses two zones and your first is within one zone. If with JAL, they use the first flight - so in your case, SGN-HND. See: https://www.jal.co.jp/jp/en/info/202...info_iata.html
The easiest way to probably find out is to call the airline up; mixed itineraries can be a hairy beast to deconstruct.
This is a more user-friendly guide to the rules:
https://www.airnewzealand.co.uk/most...ficant-carrier
- Starting or ending in the USA or Canada: If your single-ticketed journey starts or ends in the USA or Canada (this does not include journeys that only pass through the USA or Canada), the 'first marketing carrier' decides the allowance and charges that will apply for the entire journey. They can apply their airline's rules or those of the 'most significant carrier'. Where Air New Zealand is the 'first marketing carrier' we always apply the most significant carrier's rules, which may not be our own. For return trips, if you travel in a different class or have different fare products in each direction (such as Business Premier™ one way and Economy the other or seat and seat+bag), the same carrier's rules apply but the allowances may differ as they will be based on the applicable fare types and classes of travel purchased in each direction