Hanoi is 6 hours ahead of Hamburg
Best overlap: 8:00 AM to 12:00 PM in Hamburg is 2:00 PM to 6:00 PM in Hanoi.
Keep meetings inside 8:00 AM to 12:00 PM in Hamburg — that maps to 2:00 PM to 6:00 PM in Hanoi. Around 4 hours of overlap is enough to work comfortably, as long as meetings don't drift to the end of one team's day. Because Hamburg and Hanoi don't both observe daylight saving, plan to re-check the overlap around March and November.