Before entering Vietnam, most nationalities need a visa and valid passport.  Tourist visas are valid for 30 days.

There are three ways to apply for a visa:
  - through your nearest Vietnamese embassy or consulate
  - through a travel agent
  - through a visa on arrival services website

Required Documents:
  - the entrance application (printed form)
  - two 4x6 cm photos
  - a passport valid for six months after your departure from Vietnam
  - fee

Exemptions
 As of February 2009, Vietnam offers visa exemptions for citizens of Norway, Sweden, Denmark, Finland, Japan, South Korea and Russia for stays of not more than 15 days; and citizens of ASEAN countries for stays of not more than 30 days.

 People of Vietnamese origin who are living abroad and/or hold foreign passports are also eligible for a visa exemption.  You can find out how to apply for an exemption here: http://mienthithucvk.mofa.gov.vn/Support/Dosierandprocedures/tabid/136/Default.aspx

Useful links:
http://www.vietnam-visa.org/
http://vietnamembassy-usa.org/services/visa-application-process

Visa on Arrival Services:
http://www.vietnam-visa.com/
http://www.myvietnamvisa.com/