Ho Chi Minh City is 12 hours ahead of Dallas
There is no standard 8-to-6 overlap. Someone joins early or late.
Practically speaking, Dallas and Ho Chi Minh City never share office hours; 12 hours of separation keeps their days apart. To talk live, someone has to flex: an early start in Dallas can catch the tail end of the day in Ho Chi Minh City, or the other way round. Note the seasonal wrinkle: when Dallas is on daylight saving and Ho Chi Minh City isn't, the gap between them grows or shrinks by an hour.