For U.S. citizens, go to http://www.vietnamembassy-usa.org/consular/visainfo.php3 to read more about it. Here are the instructions on that website:

1. General Information
   

   A Vietnamese entry visa is required for all foreigners wishing to visit Vietnam except for citizens of countries having bilateral agreements on visa exemption with Vietnam. The Embassy of Vietnam will issue visas only upon approval by authorities in Vietnam.

   Visas can be applied by mail or in person at the Embassy of Vietnam up to six months prior to the date of travel. Applicants must summit to the Embassy of Vietnam the following:

   1. The original passport;
   2. One completed application form with one passport-size photograph stapled or glued firmly to the application form;
   3. A self-addressed stamped envelope if the passport and visa are to be returned by mail.
   The Embassy will also return passports and visas by other mailing services, such as FedEx, UPS, DHL or U.S. Postal Service Express mail. Airborne Express is not accepted because this service is currently not available to the Embassy. If these alternate mailing services are desired, applicant must provide appropriate mailing labels and account numbers. The Embassy will not be responsible for any delays or misrouted package resulting from insufficient postage, missing authorized account numbers, or the desired mailing services are not available in Washington DC. Five business days are required to process normal visa applications. Expedited applications are processed in two business days after the Embassy receives the applications (additional fee required). Please note that the Embassy is closed on Saturdays, Sundays, and certain U.S. and Vietnamese holidays.

   Please contact the Embassy of Vietnam at (202)861-2293 or (202)861-0694 for information on current visa fees.

2. Tourist Visas

   Tourist visas will be granted only for single entry with a maximum stay in Vietnam of one month.

3. Diplomatic and Official Visas

   There are no fees for Diplomatic or Official visas unless otherwise agreed upon between Vietnam and applicants country. In addition to the documents specified above, applicant must submit a Note verbal or an official letter from the concerned agencies of the U.S. federal government, foreign embassies or consulates accredited to the U.S., international organizations, or other accredited organizations based in the U.S.

4. Business or Other Types of Visas

   In addition to the documents specified in (1), applicant must obtain approval through sponsor in Vietnam. The Embassy will require five business days to process visa applications even with pre-approvals. For further information about visa procedures and fees, please call (202) 861 2293 or (202) 861 0694 or fax to (202) 861 1297 or (202) 861 0917 from 9.00 AM to 12.00 AM and from 2.00 PM to 5.30 PM (Eastern Time) from Monday through Friday.