Northern Vietnam gets real fog: valley fog in the mountains, river fog along the Red River in the mornings, and winter haze that turns Hanoi into a grey murk for days. Fog is the purest visibility problem: the speed at which you can stop must be shorter than the distance at which you can see.
Slow to your sight distance
The rule is simple and rarely followed: if you can see 30 metres, ride at a speed from which you can stop in 30 metres. In dense fog that means walking pace or stopped. The temptation is to keep moving because everyone else is — remember that everyone else cannot see either.
Be seen
Dipped beam on, not high beam: fog reflects high beam straight back into your eyes. Bright layer on top, and a reflective strip on your bag. If you stop, pull fully off the road — a motorbike stopped in a traffic lane in fog is invisible until it is too late. The legal lighting requirements are in motorbike-night-lights-vietnam.
Read what you can
In fog, sound arrives before sight: unlit bikes announce themselves by engine noise. Keep the visor closed and crack it open slightly only if it fogs from inside — an anti-fog treatment or insert fixes the recurring problem. Winter-riding-motorbike-hanoi covers the broader Hanoi haze season.
Mountain fog
Mountain roads in fog are where visibility and edge-lines both vanish: ride at a speed where the road stays inside your sight distance, and use the roadside posts, where they exist, as your lane guide. If fog on a pass thickens, stop above the road, off the tarmac, and wait.
The one-line version
Ride only as fast as you can see to stop, dipped beam on, off the road entirely if you stop — in fog, seeing and being seen are both your job, because the fog is doing neither.