Hamburg is 6 hours behind Hanoi
Best overlap: 2:00 PM to 6:00 PM in Hanoi is 8:00 AM to 12:00 PM in Hamburg.
If you set up a recurring sync, 2:00 PM to 6:00 PM in Hanoi (8:00 AM to 12:00 PM in Hamburg) is the natural home for it. Hamburg is 6 hours behind Hanoi, which gives you a solid 4-hour window. Book earlier in it and neither side is rushing. Because Hamburg and Hanoi don't both observe daylight saving, plan to re-check the overlap around March and November.