Dallas is 12 hours behind Ho Chi Minh City
There is no standard 8-to-6 overlap. Someone joins early or late.
Practically speaking, Ho Chi Minh City and Dallas 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. One seasonal catch: Dallas springs forward and falls back, but Ho Chi Minh City doesn't, so the gap changes by an hour for several months of the year.