This paper reports a study that investigated the role of prior native or first language (L1) phonological and phonetic learning on the integration of vowel quality features in the production of second language (L2) vowels by examining adult L2 Korean learners' production of Vietnamese monophthong vowels in an imitation and a read aloud tasks. Three groups of participants took part in the study (11 control Vietnamese, 11 Korean learners of Vietnamese, and 10 control Korean). The stimuli consisted of 9 Vietnamese monophthongs [foreign characters omitted] and 8 Standard Korean vowels [foreign characters omitted]. The results showed that Vietnamese vowels [foreign character omitted] and /e/ produced by Korean learners merged in vowel space, proving how a phonemic merger in L1 can influence speakers' perception and production of non-native vowels. Moreover, the three Vietnamese vowels [foreign characters omitted] produced by Korean learners in both tasks tend to cluster together. In general, the findings of this study showed that Korean learners transfer their L1 vowel quality features into the production of Vietnamese vowels.