It was pre-covid, so things may have changed since I applied, and there may be some differences for Canadians and Americans, but I'll try!
For Vietnam, I had to go to the consulate in Vancouver and submit my passport, application form, passport photo and fee. It took about 2 weeks to get the passport back with visa attached. I see there is a new website for Canadians to apply on-line, but they still send a physical visa back:
https://vietnamembassy.ca/consular-services/visa-application/
For Cambodia, I can't remember exactly, but I think we had to submit the application, passport, passport photo, and fee to an "agent" here in Vancouver, who dealt with the embassy on our behalf. I see Cambodia now uses eVisa (both E and T available).
There are visa on arrival available for both Vietnam and Cambodia, but that was not an option for us due to their restrictions at several border entry points (VoA not accepted).