Although thousand of foreigners use visa on arrival every day, how many percentage understands clearly its steps and validity? Maybe not much. The answer for the first part of the question can be found easily on the internet, take this link as an example http://www.vietnamtoursguide.com/vietnam-visa/. This visa very informative article I like, but not for the second part. Therefore, basing on my experience on this service, I would like to clarify the validity of visa on arrival.
With the visa on arrival service, the visa will be issued upon arrival at Viet Nam international borders. With each type of borders (airports, land or sea), different type of visa letter is effective and not all of websites on the internet can process all types. Most of them are dealing with the visa on arrival for traveling by air. Therefore, before applying, please read carefully the instruction on website to make sure you choose the right visa on arrival website.
In some websites, it is said the visa on arrival is not legitimate – It is INCORRECT. I was suspect this issue and asked my service provider and be surely confirmed that the visa letter is issued by Viet Nam Immigration Department and be defined in Vietnamese Immigration Laws (Ordinance on entry, exit, residence of foreigner in Viet Nam dated 28/04/2000; Decree No. 21/2001/ND-CP dated 28/05/2001; Circular No. 04/2002/TTLT-BCA-BNG dated 29/01/2002 to implement related Ordinance and Decree). Additionally, during last nearly 7 years I have used this service, there has been NOT any problem arising related visa or Viet Nam entry.
Visa on arrival is issued for 3 purposes normally: tourist (1 month), business (1 or 3 month) and family visit (1 or 3 months). You are not required to show evidence for the purpose choosing when apply, therefore, do not worry if you visit Viet Nam for the trip but receiving the 3 months visa stated your visa is for business. You can get the visa and enter Viet Nam smoothly.
Just 1 more recommendation: check if the visa on arrival is available for you. Basing on my knowledge, some countries’ citizens might not able to apply this visa. For example: Chinese living in China…
Finally, here is my visa advisor during the last 7 years www.getvisavietnam.com. Their staff is very friendly, swift-response and professional. Hope my suggestion helps when you need information regarding visa.