Not every bike in Vietnam has a rear rack — plenty of scooters and small commuters run bare tails — and a good tail bag turns a bare pillion seat into luggage capacity without drilling anything.
What they are
A tail bag straps to the pillion seat and rear frame, usually with a webbing harness under the seat or around grab rails. Sizes run from small day packs to 40-litre tourers; roll bags are the minimalist version — a cylinder that rolls closed, straps down, and doubles as a dry-ish bag for clothes (true waterproofing still belongs to a dry-bag liner — dry-bags-waterproof-luggage-riders).
Strapping it right
The physics are the same as any luggage article: heavy items low and central, the bag forward over the seat rather than hanging off the tail (carrying-luggage-motorbike-safely-vietnam). A bag perched behind the rear axle lightens the front wheel and turns the bike into a pendulum at speed. Four points of contact minimum — two forward, two rear — with the straps to frame or rails, never to plastic body panels. Elastic bungees stretch and shed; the cargo-net discipline in cargo-nets-bungee-cords-motorbike applies, but webbing with cam buckles is the proper tool for a tail bag.
The Vietnam caveats
Tail bags are fast to steal — everything soft is. Unstrap and carry the bag whenever you park somewhere you would not leave a wallet. Heat is the second issue: a black roll bag on a black seat in noon sun becomes an oven for electronics and snacks; park in shade and carry the important things in the dry bag. And if you regularly carry bulk, fit a rack and top box instead (top-boxes-racks-motorbike-vietnam) — soft luggage is for flexibility, not permanence.
Choosing one
Look for: a proper harness (not just bungee loops), side compression to stop the load ballooning at speed, rain covers or roll closure, and a carry strap for off-bike life. Test that it does not interfere with the pillion seat or the grab rails — if you carry passengers sometimes, the tail bag and the passenger cannot share the seat.