Ho Chi Minh City is 5 hours ahead of Vienna
Best overlap: 8:00 AM to 1:00 PM in Vienna is 1:00 PM to 6:00 PM in Ho Chi Minh City.
Both teams are at their desks between 8:00 AM to 1:00 PM in Vienna and 1:00 PM to 6:00 PM in Ho Chi Minh City. The shared window is roughly 5 hours; treat its midpoint as the default and you'll keep both teams happy. One seasonal catch: Vienna 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.