Filtering — riding between lanes of slow or stopped traffic — is woven into how Vietnamese cities move. Done predictably, it is normal; done recklessly, it is where a large share of city accidents happen. The legal position of lane-splitting is covered in lane-splitting-legal-vietnam; this article is about doing it well.
Speed differential is the whole game
Filter only a little faster than the traffic you are passing. As a rule of thumb, if the gap closes faster than you can stop in, you are going too fast. In stopped traffic, walking pace is fine; in moving traffic, filter only when the differential is small.
Be seen, be predictable
Ride where drivers expect bikes — along the lane seam — and watch front wheels, not car faces: a front wheel turning reveals the move a second before the indicator does. Expect mirrors and helmets to be invisible to drivers pulling out of side streets; ride as if every gap hides a motorbike coming the other way, because in Hanoi it usually does.
Pick your gaps
Gaps that disappear are the dangerous ones: beside a bus, next to a car drifting within its lane, between a truck and a barrier. If the gap feels tight, wait. Buses and trucks have large blind zones — riding-motorbike-blind-spots-vietnam covers where their drivers cannot see you at all.
Cover the fundamentals
Cover the brakes, keep the clutch in the friction zone on geared bikes, and never filter between a vehicle and the curb where doors open and pedestrians step out. Slow-speed-control-motorbike-vietnam covers the bike-handling side.
The one-line version
Filter slowly, be boringly predictable, watch front wheels, and skip any gap you cannot stop inside — the fast rider arrives minutes earlier and crashes years sooner.