To start, remember that your passport must have at least six months validity remaining beyond the date of your arrival. If not, you'll need to put off your trip until you get a new one. Assuming your passport expiration is in order, the next step is to apply for written approval to enter Vietnam. It sounds really fancy, but we did it online in a matter of minutes. There are several websites that will help you get this letter, and we chose Vietnam-Visa.com based on personal recommendations from friends and reviews we read online (and now, we can personally recommend it).
Here's what you do: Go to Vietnam-Visa.com and fill out the application form. It requires your basic information, dates of travel, and desired visa length. You pay a service fee at this point, which depends on the length of the visa you are applying for. For one month (the shortest length visa you can get), our cost was $21. The letter of approval will then arrive via e-mail within two business days. You print out that letter and bring it with you to Vietnam, along with two passport-size photos, your passport, and the remaining payment in cash (USD okay) for the stamping fee ($45 for single entry visa up to a month; $65 for 1 month multiple entry visa; and $95 for 3 multiple entry visa).
The longest part of the process for us was going to CVS and getting two passport photos taken, which is also required when applying at the Embassy. Stamping and service fees are also charged when going through the Embassy, as well as the price of postage (unless you live close enough to apply in person). You can see a further comparison between the two processes here.
For those that decide to deal with the official Embassy, there is one advantage in that once you arrive, you don't have to wait in the queue to get your visa and you can proceed directly to immigration. Despite that, we found online to be a much easier process, minus an extra 10-15 minutes at the airport, especially given the fact we didn't have to mail off our passport and the service fee for a two-day turnaround was only $21.
Got any additional tips to share? Leave them in the comments below.