Depends on Nationality and the lenght of your travel to Vietnam, you may or may not need Vietnam visa before you depart to Vietnam. But for all Nationalities, the general requirements are:
+ Your passport has at least 6 months validity from the entry date, otherwise you need to renew it before applying for a visa to Vietnam.
+ Your passport should has at least one blank page
You can check if you are entitled the Visa exemption to Vietnam. For some specific countries, you can find detail below.