For coffee drinkers who start in the Hanoi Old Quarter and end up somewhere with one ATM and no card machine — which payment method to rely on, and where each one breaks.

For anyone renting a scooter in Hanoi, Hoi An or Da Nang who wants their deposit back, their passport in their own pocket, and no argument on the last morning.

For anyone crossing to Phu Quoc or Cat Ba on a tourist visa and wondering whether the ticket window will take their card, their cash, or neither.