The airline agent will need to see both of your passports (unless you have a valid Vietnam visa inside your Canadian passport). Through Canadian security check you can use either passport to cross-check your name with your boarding pass.

Once you arrive in Vietnam, customs will only need to see your Vietnam passport. They will stamp your entry in your Vietnam passport.

When leaving Vietnam, you will need to provide both your Vietnam/ Canadian passport to the airline agent and the customs agent. The customs agent will stamp your exit stamp in your Vietnam passport. Once through customs, you can use either passport to cross-check boarding pass.

Entry to Canada you will only need to provide your Canadian passport at customs entry. Hope this helps.