Hanoi is 11 hours ahead of Washington, D.C.

There is no standard 8-to-6 overlap. Someone joins early or late.

There's no shared 9-to-5 between Washington, D.C. and Hanoi; at 11 hours apart, one city's workday is the other's evening or night. Your best bet is one team taking an early-morning or late-evening call — alternating who stretches keeps it fair over time. Because Washington, D.C. and Hanoi don't both observe daylight saving, plan to re-check the overlap around March and November.