They won't let you in without a visa.

It all depends on what country you're from. Most can get landing visas good for a month, and quite a few places (travel agencies, guesthouses) will extend them for a bit of $$$$$. The best thing to do is find a good travel agent who can get you one of the six month multi-entry visas.

if you are from member country of ASEAN (which all South-East Asian countries are) then you get automatic 30 days visa upon entry, without any extra charges. If you are not then its always good to get in advance 6 months tourist visa, from Vietnamese embassy of your country. Its usually takes only 1 week to obtain. If you are too lazy, then hope you are not from some countries, that do not have consulate/embassy of Vietnam, otherwise you can get your on arrival visa easily.