Yes, you can extend your Vietnam visa after your arrival. It takes 7 working days to extend the visa at the Vietnam Immigration Department Office in Vietnam.
Your current visa type has to be correspondent to your 'staying extending requirement'.
- Stay for 1 more month: If your current visa is 1 month single, 1 month multiple, 3 month single or 3 month multiple entry visa, you are allowed apply to extend visa for 1 more month.
- Stay for 3 more months: If your current visa is 3 month single or 3 month multiple entry visa, you can apply to extend visa for 3 more month.
You can read about Vietnam Visa Extension process through this link Click Here
Please double check with Vietnam immigration department website for any further questions and concerns about extension of your Vietnam visa.