We returned from Vietnam less than 2 weeks ago.
Required on flights.
Required at airports.
Didn't see any requirement on the train we took.
Wasn't required at any hotels of guests but some hotels required it of their employees. Both Hyatt Da Nang and Park Hyatt Saigon required it of their staff.
Maybe it's a function of where we were but I did not see a lot of people wearing them.