This study investigated the influence of first language (L1) phoneme features and phonetic salience on discrimination of second language (L2) American English (AE) vowels. On a perceptual task, L2 adult learners of English with Spanish, Japanese or Russian as an L1 showed poorer discrimination of the spectral-only difference between /æ:/ as the oddball (deviant) among frequent /ɑ:/ stimuli compared to AE controls. The Spanish listeners showed a significant difference from the controls for the spectral-temporal contrast between /ɑ:/ and /ʌ/ for both perception and the neural Mismatch Negativity (MMN), but only for deviant /ɑ:/ versus /ʌ/ (duration decrement). For deviant /ʌ/ versus /ɑ:/, and for deviant /æ:/ versus /ʌ/ or /ɑ:/, all participants showed equivalent MMN amplitude. The asymmetrical pattern for /ɑ:/ and /ʌ/ suggested that L2 phonetic detail was maintained only for the deviant. These findings indicated that discrimination was more strongly influenced by L1 phonology than phonetic salience.