Originally Posted by Mrgolfer21

Probably no way to answer this 100%, as so many scenarios can play out, but most likely you will just overnight in NRT. While it's technically not an overnight airport (you can't book a flight connecting overnight), there are times when people have to overnight. It won't be fun, you won't get lounge access, but there will be a section of the terminal blocked off for such situations. The biggest issue is the expired covid test to come back to the US...can you get another one at NRT or would you not be able to and they send you back to Vietnam (i'd personally fight tooth and nail to prevent this). Kind of up to the airline...and flying JL might have it's benefits in this case. Here's a worst case scenario that had possible chaos and definitely a lot of confusion: