To promote Vietnam tourism, the Vietnam Government has discussed and approved to allow some countries to enter Vietnam without visa, included Thailand.
If you are Thailand passport holders, you do not need a visa if staying less than 30 days. This visa exemption is approved to enter through land borders, sea ports or airports.
With the free-visa, you no need to submit any document. On arrival, you just show your passport to the Immigration Officers. They will check your nationality, and stamp onto your passport. The stamp shows that you are permitted to stay in Vietnam 30 days. It is not the visa to stick on your passport, it is just the stamp.
In case you need to stay more than 30 days in Vietnam, you should apply a visa to Vietnam in advance. Because if you use the free-visa to enter in, and then you want to stay longer, you have to extend your stay. The fee for extension is much more expensive than the fee to get a visa in advance.
In case you visit Vietnam less than 30 days but for a business purpose, you still need to apply a visa. Free visa policy is only for tourist purpose.
For those who want to learn more about Vietnam visa, please see Visa On Arrival.