You must leave Vietnam by the granted date of exit. If you wish to stay longer in Vietnam, you should apply for a visa extension.
For example:
Your Visa type is: 1 month single entry Visa
Entry date you apply: 28 Mar, 2015
>> You are allowed to enter Vietnam whenever you want from 28 Mar, 2015 to 28 Apr, 2015. And please make sure you will exit Vietnam before 28 Apr, 2015. (You can stay more in case you make your Visa extension).
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