This paper investigates the occurrence of misunderstandings in ELF (English as a Lingua Franca) interactions caused by vowel pronunciation in Brunei English. The study is based on ten audio recordings, each consisting of conversations between two participants: a Bruneian (L2 speaker) and a non-Bruneian speaker. Out of a total of 152 tokens (occurrences) of misunderstandings identified, 36 of them (23.7%) are found to involve vowel pronunciation. Data analysis includes examining vowel length, the vowels of face, goat, price, mouth, monosyllabic triphthongs, trap, vowel reduction, spelling pronunciation and American pronunciation in causing misunderstandings. The findings indicate that misunderstandings may be caused by a change in vowel length and quality in 28 tokens. The study concludes that vowel pronunciation in Brunei English conversational speech, particularly in the lack of vowel length distinction and absence of diphthongs in closed syllables may occasionally lead to a loss of intelligibility in ELF settings.