You can apply online if it is visa on arrival or you can get when you reach at the airport. Remember to carry your photos and the currency of their country. If visa on arrival not available than you have to go Visa office near you.
The process of getting a business visa or a tourist visa is generally the same. You can get the visa through a Vietnamese/Cambodia Embassy or Consulate office in your respective country. You can even get visa on arrival. Please check full information here: http://www.myvietnamvisa.com/how-it-works.html (Vietnam). For Cambodia check here: https://www.evisa.gov.kh/
For Vietnam, we applied Visa online. Once you apply it, they will send an email confirmation after nearly a week or so. Take a print copy of the mail confirmation and also take out a print copy of the Visa form from the internet. Both the forms will be needed at the airport in Vietnam on arrival.