North to south: picking the pack
A week in one region, say Hanoi with a night on Ha Long Bay, or Ho Chi Minh City with a day in the Mekong Delta, fits the 3 GB pack, which lasts 15 days. Two weeks from Hanoi down to Hoi An is a 5 GB trip, valid for 30 days.
The full run in three weeks, with the night train from Hanoi to Lao Cai for Sapa, a flight to Da Nang for Hoi An and then Ho Chi Minh City, means maps and ride apps every day: take 10 GB. If you settle in Da Nang or Hoi An for a month to work, or share the hotspot with a laptop, 20 GB is the one. Hoi An has no airport of its own, so you land at Da Nang and do the rest of the way by road. Carrying on overland? The packs stop at the border, so look at the Cambodia or Laos eSIM. The data calculator fits it to your route.