You will get it to the email you wrote in the application.
Don’t forget to print it out so the immigration officers in Vietnam can stamp your passport.
Please read “How the visa approval letter look like?” for more details about 2 type of visa approval letter : Shared visa approval letter (Multiple people sharing one letter – It’s cheap for this kind) & Private visa approval letter (one person in one letter or only your group in one letter – Extra charge is required)