Entry visas to Viet Nam can be applied BY POST or IN PERSON (by the applicant or anyone on his/her behalf) at the Embassy.
By post (Check list): Original passport, form + photo, fee in AUD (Money Order, Bank Cheque, or cash), return envelop.
In person (check list): Original passport, form+photo, cash in AUD.
Visa form is downloaded here, or in this article or in home page (Form for Consular Services). Please see detailed instructions in this article.
For the USA passport, the minimum visa's validity is up to 01-year multiple entries.