The click-nothing moment happens to everyone in Vietnam eventually: heat ages batteries fast, and the failure is usually sudden. What you do next depends on the bike and the situation.
First, make sure it is the battery
A completely dead silence at the button, with lights and horn also dead or faint, is the classic battery signature. But dash lights that come on and a healthy-sounding starter that fails to catch is a different problem — fuel, or a flooded start. Confirm by lights-and-horn before concluding battery; battery-care-motorbike-vietnam covers the longer-term diagnosis.
The push start
Most manual and semi-automatic bikes can be push-started: ignition on, in second or third gear, clutch in, get the bike rolling at a jog, release the clutch and give throttle as the engine spins. It takes a clear, safe stretch of road — not a busy Hanoi junction. Practice somewhere empty before you ever need it at night.
The important exception
Scooters with CVT transmissions cannot be push-started: there is no clutch to engage, and the rear wheel does not turn the engine. This is the single most useful thing to know about a dead scooter: it needs a jump from another battery or a replacement, not a push.
Jump-starting
Car-to-bike jump starts work with care — the car engine off, correct polarity, bike connected briefly and started, cables removed in reverse order. In Vietnam, the practical version is usually simpler: a neighbouring shop or cafe has a battery pack or a spare; roadside battery shops exist in every district. On a rental, a dead battery is a call to the shop — rental-motorbike-breakdown-hanoi covers the process — rather than roadside electrics.
The one-line version
Confirm it is the battery, push-start manuals only (never a scooter), jump with care or borrow a local battery pack — and replace it, because in this heat a revived battery is a countdown.