Ho Chi Minh City is 1 hour behind Shenzhen
Best overlap: 9:00 AM to 6:00 PM in Shenzhen is 8:00 AM to 5:00 PM in Ho Chi Minh City.
Both offices are open during 9:00 AM to 6:00 PM Shenzhen time, which is 8:00 AM to 5:00 PM in Ho Chi Minh City. The 1-hour gap barely gets in the way — you've got 9 hours of overlap, so pick whatever time fits the agenda.