Kathmandu is 1.25 hours behind Hanoi
Best overlap: 10:00 AM to 6:00 PM in Hanoi is 8:45 AM to 4:45 PM in Kathmandu.
Your window to reach both cities is 10:00 AM to 6:00 PM in Hanoi, which converts to 8:45 AM to 4:45 PM in Kathmandu. 8 hours of shared hours is more than enough — slot meetings wherever they're most convenient for the people involved.