Vietnam visa

Dear Sir or Madam: My fiance and I are travelling to Vietnam September 15 to October 5(20 days). (and we will be visiting Cambodia while we are there) We are United States citizens. We have valid passports that dont expire for 6 years. I would like to obtain a Vietnamese visa. Do I need to obtain this ahead of time or can I obtain one when I land in Ho Chi Minh airport? And if so what is the fee for that? I see that I can fill in an application and mail it. But I couldnt find what the fee was on the website for doing this. Since we are going to Vietnam and then to Cambodia and then back to Vietnam does that require a multiple visit visa? Or will a single-entry visa suffice? Also what are all the fees? It seems there both a application fee and a stamping fee? Also what about doing "visa on arrival", where you get a "visa approval letter" ahead of time? Is this so? and what are the fees with that? How do those fees compare with doing a mail in visa. And for cambodia vise can I get that when I land in cambodia (i think i will be flying there) Thank you very much for your assistance, Mark Gibson