Washington, D.C. is 11 hours behind Hanoi
There is no standard 8-to-6 overlap. Someone joins early or late.
Washington, D.C. is 11 hours behind Hanoi, which is far enough that a normal 9-to-5 in one city never reaches the other's working hours. One practical option: Washington, D.C. dials in early while it's still late afternoon or evening in Hanoi. These times hold for most of the year, but Washington, D.C.'s daylight saving (which Hanoi doesn't use) shifts them by an hour twice annually.