The tank bag is the most under-used piece of motorbike luggage in Vietnam: a small bag strapped where your torso already points, reachable at a fuel stop without dismounting. It does one job — essentials at hand — and does it better than anything else.
What belongs in one
Documents, phone, wallet, sunglasses, a small toolkit: the things you reach for every hour. Not groceries, not a laptop — load discipline is the same as any luggage (carrying-luggage-motorbike-safely-vietnam), just scaled down. A clear-map pocket on top, where phones now ride instead of paper, doubles as a navigation deck — the safer alternative to handlebar phone mounts is a tank-bag top pocket, because the screen sits lower in your sight line than phone-mounts-motorbike-vietnam position it.
Mounting systems
Magnetic flaps: fast, standard, and only for steel tanks — most Vietnamese scooters have plastic panels, where magnets have nothing to bite; straps around the steering head and under the seat are the fix, and tanks bags with a strap harness or a dedicated tank-ring mount exist for exactly this. Whatever the system, the bag must not rotate or slide when you brake: a shifting tank bag interferes with the bars, and steering inputs are non-negotiable.
Size discipline
A filled tank bag sits between you and a full-lock turn; too big and it crowds your knees at U-turns and stops. Small to medium only. When you park, the tank bag is the most-stolen piece of luggage in Vietnam — it is designed to lift off in a second, so take it with you, always; that same portability is its entire value.
Where it fits in the luggage system
Tank bag for the hourly stuff, under-seat storage for the tools, a top box for lockable bulk (top-boxes-racks-motorbike-vietnam), and a backpack only when nothing else fits — the trade-offs are argued in backpack-vs-saddlebags-motorbike. The mistake is duplicating: a tank bag plus a backpack plus a saddlebag each half-full leaves three theft targets where one well-chosen system would do.