VIETNAM
Citizens from the U.S. and Canada will need a visa to enter Vietnam. Your passport must be valid for at least six months following your arrival date and have two blank pages.
E-visas are $25 U.S. and are issued with a validity period of up to 90 days. Applications are processed in 3-5 business days, but we suggest that you apply at least two weeks prior to your departure in case of delays.
Here is a link to information about the online application for the e-Visa and here are instructions and the application.
You will need to upload a photo of your passport data page as well as a passport photo.
CAMBODIA
U.S and Canadian citizens can get the Cambodia Visa on arrival at the airport in Siem Reap. You will need to bring: your passport valid for at least 6 months with two empty pages in addition to the two needed for Vietnam; one 4x6cm passport photo; your onward flight ticket (which your tour leader will have) and $25 U.S. cash to pay for it. Easy!