Helmets are legally required for riders and passengers in Vietnam — helmet-law-vietnam covers the rule and the fines — but wearing one only matters if it fits and fastens. Most helmets sold cheaply on Vietnamese sidewalks are shells with foam; most helmet failures are fit and strap failures.
What to look for
A helmet works by managing an impact through its shell and crushable liner. Look for a recognised standard mark inside — any of the major national standards is the honest baseline; helmets with no mark at all are decoration. Full-face protects the jaw and face, open-face is the common city compromise, and the half-shell style is better than bare skin and not much else. For city riding at Hanoi speeds, a decent open-face with a visor is a reasonable daily choice; for open roads, full-face.
Fit is everything
Put it on: it should be snug enough that shaking your head moves the helmet, not the head inside it, and the cheek pads should press slightly. The strap fastens under the jaw snugly enough to fit two fingers, no more. A helmet that is comfortable in the shop is usually one size too big — it must be snug while stationary.
Children need real helmets
A child on a pillion needs a properly fitted child helmet, not a loosened adult one — child-passengers-motorbike-vietnam covers the legal side. Child helmets exist in every Vietnamese market; check the fit the same way.
Life and replacement
Helmets are one-impact items: after any hit — even a drop onto concrete — the liner is done. Foam also ages: a helmet used daily in Hanoi’s heat and humidity should be replaced after a few years regardless of crashes. What to wear with it is covered in what-to-wear-riding-motorbike-vietnam.
The one-line version
A marked standard, snug enough that your head moves the helmet, strap under the jaw — replaced after any impact or a few years of daily sun.