Prague is 5 hours behind Hanoi
Best overlap: 1:00 PM to 6:00 PM in Hanoi is 8:00 AM to 1:00 PM in Prague.
Plan around 1:00 PM to 6:00 PM in Hanoi, which is 8:00 AM to 1:00 PM in Prague, to catch everyone during office hours. Since Prague is 5 hours behind Hanoi, the workdays overlap by around 5 hours; 1:00 PM in Hanoi (8:00 AM in Prague) makes a reliable recurring slot. For part of the year the maths changes: Prague observes daylight saving, Hanoi does not, shifting the overlap by an hour.