Montreal is 11 hours behind Ho Chi Minh City
There is no standard 8-to-6 overlap. Someone joins early or late.
Because Montreal is 11 hours behind Ho Chi Minh City, the two offices are essentially never open at the same time. If a live call is essential, aim for the very start or very end of a workday in one city and accept it'll be off-hours for the other. Don't be caught out by clock changes — Montreal observes daylight saving and Ho Chi Minh City doesn't, so the offset varies by an hour through the year.