Hanoi is 12 hours ahead of Ottawa
There is no standard 8-to-6 overlap. Someone joins early or late.
Ottawa and Hanoi live on different parts of the day. With 12 hours between them, standard working hours simply don't meet. Make the most of handoffs — end-of-day in one city becomes start-of-day in the other, so pass work along rather than meeting live. For part of the year the maths changes: Ottawa observes daylight saving, Hanoi does not, shifting the overlap by an hour.