Summarize the following:
If you live in an approved country, you can apply for a visa online and wait for them to mail you the letter. Go to the Vietnamese Immigration Department's website, follow the instructions for your specific country, and fill out the application online to apply. The costs are the same for online applications, but you'll pay an additional $25 for online processing. To begin the process, visit the Vietnamese Department of Immigration online at https://evisa.xuatnhapcanh.gov.vn/. There are several online agencies that approve visas and do the paperwork for you, but they'll upcharge you for your application. It's best just to go through official channels. The Vietnamese dong is Vietnam's national currency, and you have to change some cash out if you plan on buying anything. Go to your bank and exchange some cash before flying out to avoid getting ripped off at a currency exchange. You probably won't spend more than $25 a day, even if you eat out for every meal. Many places in Vietnam do not take credit cards, so you may not be able to get everything you need with plastic. The dong is an extremely unstable currency, so things are pretty cheap in Vietnam if you're converting American dollars, British pounds, or Euros.
Apply for a visa online if you don't mind waiting a little for approval. Change your money out at a bank before flying.