Vietnam's days as a pariah state, cut off from the rest of the world are long gone. Today the country boasts numerous land crossings with all of it's neighbours and international flight connections to Australia, Cambodia, China, France, Germany, Hong Kong, Laos, Malaysia, Russia, Singapore, South Korea, Taiwan, Thailand and the USA (among others). You can also enter by boat from Cambodia and by train from China.
You'll need a passport with at least six months validity to enter Vietnam.
The vast majority of foreign tourists enter Vietnam on a tourist visa. This visa must be issued before arrival and the typical visa is valid for 30 days. Depending on the consulate that issues the visa it begins to expire from the day it is issued, so it is a good idea to get your visa as close as possible to your intended arrival date. Extension of Vietnamese visas is possible once you are in Vietnam.
Nationals of Indonesia, Laos, Malaysia, Philippines, Singapore and Thailand do not require a visa if they plan to stay no longer than 30 days in Vietnam.
Nationals of Denmark, Finland, Japan, Norway, South Korea and Sweden do not require a visa if they plan to stay no longer than 15 days in Vietnam.