As mentioned above, UK citizens get a visa exemption.
As of 1 Feb 2017, US citizens can apply online for an e-visa....no more letters and VOAs, unless you want to spend 30-45 mins waiting for your VOA to be processed at the airport. My e-visa took less than 24 hours to obtain. Cost is $25 for a 30 day, single entry visa.
Go here to apply for the e-visa:
https://evisa.xuatnhapcanh.gov.vn/