Visa is NOT required for citizens of South Korea to stay in Vietnam for less than 15 days. If you intend to stay in Vietnam for longer than 15 days, then you have to apply for a visa and may have 2 options: Visit Vietnam embassy in South Korea (Seoul) or apply for a visa on arrival to Vietnam from South Korea online. Korean citizens may apply for both tourist and business visa to Vietnam for the stay of 1 to 6 months from South Korea. Please check this link for visa fee for Korean citizens.

All the information for getting a visa from Vietnam embassy in South Korea can be found here: http://www.vietnamembassy-seoul.org/