Hanoi winters are not Scandinavian, but 12 degrees with damp wind at 40 km/h strips heat from hands faster than standing still ever could, and cold hands are slow hands — fumbling levers is a safety problem before it is a comfort one. Heated grips are the direct fix.
What they are
Resistive heating elements either moulded into replacement grips or wrapped under your existing grips, wired to the bike’s 12V system through a switch, usually with two or more heat levels. Fitted properly they hold your hands at comfortable function even on the dampest mornings.
The physics of warm hands
Grips heat your palms; the backs of your hands still face the wind — which is why heated grips plus a windproof glove is the combination that works, while heated grips alone disappoint riders in fast cold air. The glove-side logic (insulated, windproof, still able to feel the levers) is in riding-gloves-vietnam-climate, and the grip choice itself interacts with the heated version — see handlebar-grips-choosing-motorbike.
Installation realities
It is an electrical job: a proper fused connection, clean handlebar surface, correct throttle-side clearance so heating does not bind the throttle tube, and routing that does not snag at full steering lock. Done badly, the failure modes are the electrical faults covered in simple-electrical-faults-horn-switches-fuses; and on a rented bike this is a wiring modification — ask the owner first (see rental-motorbike-modifications). Power draw is modest but real; on a small battery the habit in battery-care-motorbike-vietnam matters, especially with the cold-start demands of winter (cold-start-motorbike-winter-vietnam).
The honest alternative
Many riders solve Hanoi winters with good gloves and handlebar muffs (full windproof covers) — cheaper, zero wiring, and dramatically warm, at the cost of looks and slightly bulky lever feel. Muffs plus winter gloves cover most commuters; heated grips are for riders who want the luxury version. The full winter-riding picture is in winter-riding-motorbike-hanoi.