Vietnam is a bit stricter when it comes to entry than its neighbors Cambodia and Laos, and even Thailand. Almost all travelers are required to have a visa to enter Vietnam, and visas on arrival are not available for Vietnam.

For air entry to Vietnam, tourist visas can be obtained ahead of time online, but there is still no visa on arrival even at international airports. Visa on arrival online, is just a pre-approval letter, and you still need to get the visa at the airport by using the pre-approval letter. The visas are from $17 for a single entry, one-month visa, to $30 for a three-month multi entry visa. See http://vietnamvisa.govt.vn/

• There are many scam sites offering you visa on arrival or online service. Only use the official government’s website listed above.
• A visa online is not valid for a land crossing, only arrival by airport.
• The online visa is not a visa, it is just an visa approval letter, the official visa will be given to you at the airport.
• It is difficult and expensive to extend a visa once you are in Vietnam.
• Get a 3 months multi-entry visa if you think you will be staying longer in Vietnam or might hop over to Laos and Cambodia and then return to Vietnam.
• Cambodia is the easiest place to get a Vietnam Visa. Most large hotels and travel agents can assist you.
• Only use large travel agents such as World Express Tours (See http://www.worldexpresstour.com) in Cambodia, i have used Okay Guesthouse in Phnom Penh with excellent service, http://www.okay-guesthouse.com. I have no affiliation and get no commission if you use these services, they are just what I and many locals use.
• The start date of your visa is the day that you say you will enter Vietnam on your application, and not the day that you actually enter. Thus get the dates right. If you enter after the start date of your visa, you will lose those days, and you cannot enter before the start date of your visa.