This study examined intelligibility of twelve American English vowels produced by English, Chinese, and Korean native speakers in quiet and speech-shaped noise in which vowels were presented at six sensation levels from 0 dB to 10 dB. The slopes of vowel intelligibility functions and the processing time for listeners to identify vowels were dependent on speakers' language backgrounds and non-native speakers' vowel intelligibility in quiet. These results indicated that noise background affected non-native speakers' vowel intelligibility more greatly than native speakers, possibly due to the acoustic deviations in non-native speech and lack of listeners' experience to non-native produced speech.