Apple Holiday Calendar for Vietnam
Okay, Apple had made it so that you have to change your region in Language and Region Preferences in Settings to Vietnam. Also, you need to add the regional language as well to see the holidays of that region, but you don't need to make it the primary language. View here for more information.

There isn't a link for apple's calendar subscription, so the easiest is to just use another calendar for Vietnamese holidays: Vietnamese Calendar.

Hope this helps ;D