Hanoi is 6 hours ahead of Geneva
Best overlap: 8:00 AM to 12:00 PM in Geneva is 2:00 PM to 6:00 PM in Hanoi.
For the smoothest call, aim inside 8:00 AM to 12:00 PM in Geneva, i.e. 2:00 PM to 6:00 PM in Hanoi. Since Hanoi is 6 hours ahead of Geneva, the workdays overlap by around 4 hours; 8:00 AM in Geneva (2:00 PM in Hanoi) makes a reliable recurring slot. Don't be caught out by clock changes — Geneva observes daylight saving and Hanoi doesn't, so the offset varies by an hour through the year.