Visa extension in Vietnam is very convenient, but I recommend to do it from the major cities like Hanoi, Dalat, and Saigon (the latter is highly recommended). You can get it from the travel agencies from $20 USD for one month extension, $55 USD for 3month single-entry visa, and $85 USD for 3 months multiple-entry visa easily for atleast 5 working days processing. Saigon is the best place to do it, specially for the ASEAN members who’s under the ASEAN visa free for 21-30 days category.
(Image from google images)
Other towns that do visa extensions for ASEAN members is not recommended. They charge a lot, they say it is because you don’t have a visa so they are charging $105 USD to extend your visa for a month. Saigon does it simple and as ordinary as other country citizens extending their visas, no questions asked.