Getting a Vietnam visa extension is fairly easy when doing it via a travel agent as they save you the leg work. So your travel agent will send off your application for a visa extension to either the immigration office in Ho Chi Minh City, Nha Trang or Hanoi (takes about 7 days). How much you pay for your Vietnam visa extension depends on several factors such as where your visa was initially issued and how many times you have extended your visa.
Yes, getting a visa extension while in Vietnam isn't difficult but you should do it as soon as you arrive so don't leave it until the last minute. Also, remember that you can only extend your visa as far as you did originally. In other words, if you have a visa for 30 days and want to extend it, you will only get a 30 extension.
best way to get visa extension in Vietnam I couldn't too stress it more! Make sure you do your visa extension before it expires to avoid a world of trouble (it can take up to 10 days). Also, like the OP says, leave the visa extension work to the travel agent. You can do it yourself but it is not worth the hassle.
Vietnam Visa Extension tip if it is the first time you are renewing the visa, it will cost you about $30 (plus a handling fee) and takes 3-4 working days to process (Monday to Friday). The maximum period you can ask for is 30 days and it costs the same whether you ask for 1 day or 30 days so you may as well request 30 days just in case.