Only citizens of certain countries can visit Vietnam without an entry visa, specifically as follows:

* Not more than 30 days: for citizens of Thailand, Malaysia, Singapore, Indonesia and Laos.

* Not more than 21 days: for citizens of Philippines.

* Not more than 15 days: for citizens of Japan, South Korea, Sweden, Norway, Denmark, Finland, Russia and Cambodia.

* French citizens holding valid diplomatic passports are exempt from visa requirements when visiting Vietnam and are allowed to stay for up 3 months at one time or on several visits within six months since their first immigration dates.

* Citizens of Chile holding valid diplomatic or official passports are exempt from needing entry, exit and transit visa in Vietnam’s territory and are allowed to stay for up 60 days on each visit.

Citizens from all other countries are required to get an Entry visa vietnam before departure or a pre-approved letter (call as visa on arrival and issued on arrival at Vietnam’s International Airports) supplied before arrival in Vietnam.

For more information:
https://getvietnamvisa.com
http://www.hotels-in-vietnam.com/vietnam/vietnam-visa.html