To prevent any problem, after the officer returns your passport, you need to check carefully there are 2 stamp in your passport was issued or not.
1. The Entry stamp. This stamp will show the date you entered Vietnam
2. Permitted numbers of days you are allowed to stay in Vietnam. The stamp says that you are permitted to stay until… day … month … year.
It means you can stay in Vietnam for number of days which is said in the stamp and you must exit Vietnam before the end-day unless you extend your Visa.
* When you exit Vietnam, an Exit stamp will be issued into your passport next to the Entry stamp.
Please check all these information carefullly before you leave the Custom counter at the airport.
Hope you have a great trip to Vietnam!