This question is an exact duplicate of:
I am a Vietnamese passport holder. I'm going back to Vietnam from Seattle next week. I will have a half-day layover in Incheon Airport. I want to go into the city - Seoul just for resting, like getting a hotel and stuff. Do I need a Korean visa for this? If I do, where should I go and get it? How long will it take?
My U.S. visa is still valid. Also, I booked tickets from Vietnam to US already.