The street-stall SIM costs triple and dies early

An "unlimited" SIM from a street stall runs 200,000–300,000 VND (~$8–12) — two to three times what the same connectivity costs at an official carrier store — and it has a habit of quitting early. The two failure modes: data that depletes far faster than advertised, or an expiry clock that started before you bought it. Either way you're back at a counter buying data twice.