Alright, based on the discussion above (thanks everyone!), I went for:
opening_hours = Mo-Su 10:30-16:00+ open "Bán hết là đóng cửa; đóng cửa vào mùng 1, 15 âm lịch."
opening_hours:en = Mo-Su 10:30-16:00+ open "Closes when sold out. Closed on the 1st and 15th day of the lunar month."
This is simple and readable, provides “base” opening hours when the shop can realistically be expected to be open (10h30-16h), but also adds extra context on the open-ended hours (via + and comment).
Unfortunately there is no support for alternate calendars so the regular closure on the 1st and 15th of the lunar month are only indicated as comment.
The comment is Vietnamese-first and an English version is under opening_hours:en.