Keeping your French SIM card does not mean you should use your French plan normally in Vietnam.
Vietnam is outside the European roaming zone.
Depending on your operator and plan, mobile data, calls, and SMS can therefore be expensive.
Before departure:
disable data roaming on your French SIM card.
Then configure:
Mobile data → Vietnamese SIM
French SMS/calls → French SIM when necessary
This avoids an unpleasant surprise on your bill.