Mobile internet is essential for Grab, Google Maps, and translation. Two options: eSIM before departure or physical SIM at the airport.
eSIM (recommended)
Activation before landing. Airalo and Holafly offer 7 to 30-day plans. Expect to pay 8 to 20 € for 10-20 GB.
SIM at the airport
Viettel and Vinaphone have stands in Hanoi and Ho Chi Minh City. ~10 € for 30 days / unlimited data (fair use). Passport required.
Tips
Check if your phone is eSIM compatible. Keep your French SIM in the secondary slot if possible.