I'm currently an international student studying in the US. I'm Vietnamese and have an F-1 visa. I'm planning to take a round trip from Vietnam to Korea this summer and I was wondering about the visa requirements when I go.
While there are exemptions for those with a visa issued by certain countries, you must be en route to or returning from the country that issued that visa (or a third country). A round trip from your home country does not exempt you, and you would need to obtain a visa. You'll find the visa details and documents list on Korea's Consulate in Vietnam website.
You could make use of the 30-day transit facility, if (and only if) traveling on a three-way ticket: USA -> ICN -> VN. That will allow you to get a visa waiver up to 30 days.