I'm Vietnamese and now studying in the US. I also have a Vietnamese passport and US visa. If I come from San Francisco to Vietnam and I have to transit in Korea 3 days, can I go outside to travel with 3 days, then come back to Korea airport, and go to my country - Vietnam ? Thank you.
As stated by Timatic, the database used by airlines:
Nationals of any country with a visa issued by the USA if in transit through Korea (Rep.) if: -holding confirmed onward tickets on flights departing within 30 days; when -arriving from the country that issued the visa (a visa that expired on departure from that country is accepted) and departing to a third country.
So yes, you can enter South Korea by presenting your passport and connecting flight ticket and stay for max 30 days