To promote Vietnam tourism, the Vietnam Government has discussed and approved to allow some countries to enter Vietnam without visa, included South Korea.
If you are South Korea passport holders, you do not need a visa if staying less than 15 days. This visa exemption is approved to enter through land borders, sea ports or airports. However, a gap of at least 30 days between two visa-free visits is required. It means that if you enter Vietnam with a free-visa, then come to another countries and come back to Vietnam less than 30 days, you cannot use free-visa again.
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 15 days. It is not the visa to stick on your passport, it is just the stamp.
In case you need to stay more than 15 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 are not fix about your itinerary, you should consider about applying a visa in advance. Some people come to Vietnam, then cross the border to Cambodia, Laos, China or fly to Thailand, Singapore, Malaysia, etc. and want to come back Vietnam to continue their experience in Vietnam, but they are not allowed because the gap at least 30 days between two free-visa visits is required. Not many tourists know this issue, so please carefully study about your itinerary before deciding to use free-visa or apply a visa.
In case you visit Vietnam less than 15 days but for a business purpose, you still need to apply a visa. Free visa policy is only for tourist purpose.