From http://www.immigration.go.kr/HP/IMM80/imm_04/imm_p01/vm2.jsp:
Visa-free Entry for tour and transit
Nationals of the countries below can enter Korea without a Korean visa for a stay of up to 30 days, if their purpose of visit is tour or transit
Asia
MACAO, BRUNEI, SAUDI ARABIA, U.A.E, YEMEN, OMAN, JAPAN, QATAR, TAIWAN, H.K(90days), KUWAIT
※ Only Diplomatic·Official passport : INDONESIA, LEBANON
North America
U.S.A, CANADA(6months)
South America
GUYANA, GUATEMALA, VENEZUELA, ARGENTINA, ECUADOR, HONDURAS, URUGUAY, PARAGUAY
Europe
MONACO, Vatican, BOSNIA HERCEGOVINA, Cyprus, SAN MARINO, Serbia & Montenegro, SLOVENIA(90days), ANDORRA, ALBANIA, CROATIA
Oceania
GUAM, NAURU, NEW CALEDONIA New Caledonia, MICRONESIA, SAMOA, SOLOMON IS, KIRIBATI, FIJI, AUSTRALIA(90days), MARSHALL IS, PALAU
Africa
SOUTH AFRICA, Mauritius, Seychelles, SWAZILAND, EGYPT
(Note that Vietnam is not on the list.)
Visitors meeting the following requirements also can enter Korea without a Korean visa for a stay of up to 30 days, if their purpose of visit is transit
Note) Nationals of Cuba,Macedonia ,however, must apply for korean visa, even if they meet these requirements
Holding a visa or a re-entry permit of the U.S. Japan, Canada, Australia, or New Zealand (5 countries)
Travelling from either of the five countries to a third country via Korea, or travelling from a third country to either of the five countries via Korea
Holding a confirmed onward flight ticket for departure within 30 days
No Immigration offence records, while in the five countries
Those who have visited Korea more than 4 times in recent two years, or those who have visited Korea more than 10 times in total can enter Korea without a Korean visa, notwithstanding the requirements above
Note) However, this facility for frequent visitors is not applicable to nationals of "China (P.R.C)", "Cuba" , "Macedonia"
It appears that you are eligible for transit without visa if you have no past immigration offences in any of the five countries.