4. Get visa stamp at Vietnam Airport
Put all above mentioned things along with your passport in a package, then show them to Immigration Officers at Vietnam arrival airports.
The Approval Letter should be adequate to prove that you are allowed to entry Vietnam if it is one of the boarding requirements from your airline. Upon arrival to Vietnam International Airports (Hanoi, Da Nang or Ho Chi Minh city), you present all things inside the prepared package to Immigration Officers to get your visa stamped.