Hanoi is 6 hours ahead of Copenhagen
Best overlap: 8:00 AM to 12:00 PM in Copenhagen is 2:00 PM to 6:00 PM in Hanoi.
Shoot for 8:00 AM to 12:00 PM in Copenhagen; your Hanoi colleagues will see that as 2:00 PM to 6:00 PM. With 4 hours to play with, a mid-window meeting keeps things balanced for Copenhagen and Hanoi alike. Note that Copenhagen follows daylight saving while Hanoi stays put, so the times above drift by an hour seasonally.