Every rider carries things, and the carrying method is not a neutral choice: a backpack changes your centre of gravity, your heat, and what happens in a fall; saddlebags change the bike instead. The Vietnamese daily-carry question deserves an honest comparison.
The backpack case
Strengths: portable off the bike (the laptop goes with you into the café), cheap, nothing to mount, nothing to leave on a parked bike. Weaknesses, Vietnamese edition: a loaded backpack raises weight high on your spine and pulls you back in the tuck; in the heat it is a personal sauna; loose straps near controls are a real hazard; and in a fall, a hard object in a backpack becomes a back-shaped injury. The fit discipline that makes a backpack tolerable — snug, positioned high, straps secured, heavy items low and central — is part of the general load-carrying rules in carrying-luggage-motorbike-safely-vietnam.
The saddlebag case
Strengths: weight sits low and the rider is free; bigger capacity; weatherproof options exist for the rainy season; and the bike, not your spine, does the work. Weaknesses: mounting needs racks or a solid rear (soft bags need the discipline of keeping them clear of the exhaust and rear wheel — a melted or wheel-jammed bag is a classic first-timer failure); they tempt overloading (see the load limits in carrying-luggage-motorbike-safely-vietnam again); and on a parked bike they, and their contents, are exposed — the lockable alternative is covered in top-boxes-racks-motorbike-vietnam.
The hybrid most riders settle on
Small daily items on the body, bulk on the bike: a slim pack or jacket pockets for the phone, wallet and keys (the full everyday-carry list is in what-to-carry-riding-motorbike-vietnam), and bags or a box for groceries and gear. Long-distance commuters and touring riders go further: vented riding jackets — riding-jacket-vietnam-climate — replace the “carry a jacket” backpack entirely, which is half the backpack problem solved at the source.
The safety asymmetry
The deciding argument is the fall one: in a low-speed Vietnamese street spill, a backpack with hard contents is the thing between your back and the road, and not in a good way. Bags on the bike keep the crash energy off the spine. For anything heavy or rigid, bike-mounted luggage wins; for the light and essential, the body is fine.
Carry the small stuff on you, the heavy stuff on the bike — the conclusion the everyday Vietnamese rider reaches within a month.