Again, please confirm that it is a 320/321 roster.
Is it possible that the 3 days in Hanoi is actually because he ended in Hanoi and was positioned back to Saigon? Then he is positioned back to Hanoi 3 days later to again start flying?
This happened to me once or twice on the B767. Occasionally I spent 48 hrs in Hanoi because they liked the extra "Standby"(reserve) coverage. But never 3 days.