Washington, D.C. is 11 hours behind Ho Chi Minh City
There is no standard 8-to-6 overlap. Someone joins early or late.
Because Washington, D.C. is 11 hours behind Ho Chi Minh City, the two offices are essentially never open at the same time. If a live call is essential, aim for the very start or very end of a workday in one city and accept it'll be off-hours for the other. For part of the year the maths changes: Washington, D.C. observes daylight saving, Ho Chi Minh City does not, shifting the overlap by an hour.