Motorbike theft in Vietnamese cities is mostly a crime of convenience: a bike that is easy to roll away, in a spot nobody watches, on a night nobody passes. Security gear works by breaking that convenience — and layers beat gadgets.
Disc locks: the honest baseline
A disc lock clamps the front brake disc so the wheel cannot roll; many include a reminder cable to the handlebar so you do ride off with it engaged (a memorable low-speed lesson). Disc locks deter casual roll-away thieves and buy time, but a determined crew with a van defeats any portable lock eventually — the lock is a deterrent layer, not a vault. Placement discipline matters: through the disc, not the spokes; keyed and checked every time; and the same key every ride so it is not a chore. For rental riders: locks that attach to the rental bike usually need the shop’s consent, and theft/liability boundaries are a rental-agreement matter — see theft-loss-responsibility-rental-motorbike-hanoi.
Chains: the anchor argument
A chain’s value is anchoring: through the frame or a wheel, around something fixed — a post, a proper ground anchor, a heavy structure. The classic failure is chaining the wheel to itself (the bike rides away on the other wheel) or to something removable. Chains are heavy and live at home or in a top box; they shine for overnight parking at fixed places. Locking the helmet to the chain too is a bonus — helmet-locks-theft-vietnam covers the rest of that problem.
Where you park is still the biggest lock
All gear combined loses to parking badly: quiet dark spots with no passers-by are the theft buffet. Watched parking lots, building basements and busy, lit streets shift the odds more than any product — the terrain is mapped in where-to-park-motorbike-hanoi. The audible layer (alarms) is covered in motorbike-alarms-vietnam; combine rather than choose.
The layered summary
Deterrence is arithmetic: disc lock + chain to something fixed + decent parking + (optionally) an alarm. Each layer removes some fraction of the convenience that theft depends on. No single product is a promise; the stack of boring habits is the security system.
Thieves take the easy bike — your job is to never be the easy bike.