In Vietnamese traffic, a pillion with a loose or borrowed hat is a daily sight and a daily misunderstanding: passengers need real helmets as much as riders, and the law expects them too.
The rule that is not negotiable
Every person on the bike wears a properly fastened helmet — the physics does not distinguish between the front seat and the back seat, and neither does the pavement. A passenger’s helmet must be a helmet, not a costume: the decorative shell problem is exactly as dangerous for a pillion. Rental riders should start here: ask what the rental includes for a second rider — what Hanoi rentals typically include around helmets is covered in do-hanoi-rentals-include-helmets, and anything the shop provides must still pass your own fit and fastening check.
Sizing someone who is not there
A spare passenger helmet should be sized to the person who will actually wear it most often — bring them along to try it, because head shape varies and helmet-fit-guide-vietnam applies to passengers exactly as to riders. If the spare must fit “whoever climbs on”, choose an adjustable-pad model and check the strap system every single time: a strap left loose converts a good helmet into a flying object.
Carrying the spare
A second helmet spends much of its life attached to the bike rather than a head. Hooked on the mirror in central Hanoi, a decent helmet walks away — the practical anti-theft habits are in helmet-locks-theft-vietnam, which covers locks, carrying options and where not to leave it. Inside a top box or under the seat (on scooters with the room) beats any external hanger.
Riding two-up well
The gear is half the job; the riding technique for carrying a passenger — passenger positioning, braking distances, communication — is covered in balancing-pillion-riding-motorbike-vietnam. A pillion in a good helmet, seated properly and briefed, is the difference between a relaxed two-up ride and a rolling argument.
Buy the second helmet as carefully as the first: the back seat deserves the same protection as the front.