Hanoi is 10 hours ahead of Santiago

There is no standard 8-to-6 overlap. Someone joins early or late.

The 10-hour offset between Santiago and Hanoi means there's no point hunting for a 9-to-5 overlap — there isn't one. Expect compromise: someone in Santiago or Hanoi will need to take the call outside the usual 9-to-5, so share that load. Finally, Santiago observes daylight saving and Hanoi doesn't, so this window shifts by an hour for part of the year — recheck it each spring and autumn.