A motorbike carries best when loaded like a backpack: low, centred and snug. The same bag that vanishes under your seat changes the entire machine when hung on the handlebars. What to carry day to day is covered in what-to-carry-riding-motorbike-vietnam; this is about how to carry it.
The order of storage
Under-seat storage first, then a top box or rear rack, then a tank bag or cross-body bag worn by you. The worst places are handlebars (steering goes heavy and vague), the very back of the rack unstrapped (swings the rear in corners) and anything hanging from a hook off your body. A heavy item should sit as close to the bike’s centre as possible.
Secure it like it will fall
Because it will. Straps over bungee cords: cords snap and launch bags into traffic. If you use a rear box, check its mount occasionally — the vibration of Vietnamese roads loosens everything eventually.
Weight changes the ride
A loaded bike brakes later, corners wider and wallows at speed. Increase following distance, brake earlier, and reduce speed on bends — the same adjustments as carrying a pillion, covered in balancing-pillion-riding-motorbike-vietnam. For day trips with real luggage, choose a bike suited to the load; best-motorbike-for-day-trips-from-hanoi goes through that choice.
Long arms, low mirrors
Bulky loads widen your profile. Expect to catch a bag strap on a mirror or a parked bike’s handlebar; narrow loads also protect the bike in the tight alleys of the Old Quarter, where anything wide will scrape eventually.
The one-line version
Load low and central, strap it like it will try to escape, and ride as if the bike weighs what it now actually weighs.