Would there be any issue with translating the comment to Vietnamese? (“Bán hết là đóng cửa; đóng cửa vào mồng 1, 15 âm lịch.” Something like that.) I suppose we could preserve the English translation in opening_hours:en=*, in case this restaurant is also popular with English-speaking tourists.
It’s unfortunate that there isn’t better syntactic support for alternative calendars. There’s a fair amount of off-the-shelf software for converting to and from the Vietnamese calendar.