There is no 9–6 overlap. Stretching to 07:00–21:00 on both sides gives you 17:00–21:00 San Francisco time:
17:00 you → 07:00 Hanoi next day
18:00 you → 08:00 Hanoi next day
19:00 you → 09:00 Hanoi next day
09:00→23:00outside Hanoi working hours
10:00→00:00next day · outside Hanoi working hours
11:00→01:00next day · outside Hanoi working hours
12:00→02:00next day · outside Hanoi working hours
13:00→03:00next day · outside Hanoi working hours
14:00→04:00next day · outside Hanoi working hours
15:00→05:00next day · outside Hanoi working hours
16:00→06:00next day · outside Hanoi working hours
17:00→07:00next day · outside Hanoi working hours
Working hours assumed 09:00–18:00 local. Offsets include daylight saving where it applies, so this shifts through the year.