Beijing is 1 hour ahead of Hanoi
Best overlap: 8:00 AM to 5:00 PM in Hanoi is 9:00 AM to 6:00 PM in Beijing.
For the smoothest call, aim inside 8:00 AM to 5:00 PM in Hanoi, i.e. 9:00 AM to 6:00 PM in Beijing. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem.