Adelaide is 2.5 hours ahead of Hanoi
Best overlap: 8:00 AM to 4:00 PM in Hanoi is 10:30 AM to 6:30 PM in Adelaide.
Hanoi and Adelaide overlap from 8:00 AM to 4:00 PM (Hanoi) to the equivalent 10:30 AM to 6:30 PM in Adelaide — that's your meeting window. 8 hours of shared hours is more than enough — slot meetings wherever they're most convenient for the people involved. One seasonal catch: Adelaide springs forward and falls back, but Hanoi doesn't, so the gap changes by an hour for several months of the year.