Melbourne is 3 hours ahead of Hanoi
Best overlap: 8:00 AM to 3:00 PM in Hanoi is 11:00 AM to 6:00 PM in Melbourne.
Your window to reach both cities is 8:00 AM to 3:00 PM in Hanoi, which converts to 11:00 AM to 6:00 PM in Melbourne. 7 hours of shared hours is more than enough — slot meetings wherever they're most convenient for the people involved. A quick caveat — Melbourne's clocks move for daylight saving and Hanoi's don't, so the times here are right for only part of the year.