For UK citizen, you will get Visa exemption for 15 days to Vietnam so no need to apply VOA letter. US citizen you can get VOA letter online, it is quite cheap - we paid 9 USD for single entry then pay stamp fee at airport 25 USD - only need to fill on their VOA, then they will send you back VOA letter, print it out and show at immigration. You can check out with: http://indochinabesttravel.com/en