The short answer is to get the Vietnam Visa when you purchase the ticket at the travel agency where you bought the ticket, the ticket should bundle with the Vietnam visa included. However if you bought the airfare ticket online like expedia then the visa is not included.
The Vietnam visa is required for all USA citizenship visitors. Over the years the price has gone up from $25 to $135 then back down to about $90 it fluctuates constantly so don’t be surprise if one day your travel to Vietnam surprise you with a $200 visa one month single entry. No one really know the price of Vietnam visa until the day you submit it and the Vietnam embassy tells you how much you need to pay in order for them to process it.
So the best way to get the Vietnam Visa is through agency where you bought the Vietnam airfare ticket. Second is go directly to the Vietnam embassy website in the usa like http://vietnamembassy-usa.org/ yes it legit, and be careful that site is not https or ssl secure site, so it is possible that attackers hackers may steal your data if your browser is not updated and protected, however it is a real legit website for the usa embassy vietnam no worries there. They have many ways you can get the visa, but the price pretty much the same going through them. You can mail, fax, email, or online application. Online application is the most convenient, just fill out the form, upload your photo and passport, then they tell you how much you need to pay, and within 3 days wallahhhh your visa book sticker mailed to you overnight probably fedex.
There are many other websites out there that offer Vietnam visa cheaper, but be careful of scammer. Those other legit sites that offer Vietnam visa cheaper than the direct method through vietnam embassy might offer you something so call Vietnam On Arrival visa (VOA), although it’s cheap saving you a few bucks, but what you get is an approval letter then you need to stop by some office in Vietnam airport to get the actual stamp before you can enter immigration custom clearance area, sometime it’s fast, but most of the time it’s really slow like up to few hours, so be careful if you have children or old age people traveling with or simply can’t stand the waiting line. Go with the direct method through Vietnam embassy in many locations in USA.