The vowel space is a construct used to describe how well a speaker differentiates their vowels. Ideally, the more differentiated a speaker’s vowels, the easier time listeners will have in understanding that speaker. This work is an examination of how the vowel space relates to single word intelligibility tasks, narrative expression, subjective ratings, and referential communication using a barrier task. This work describes a study using one native speaker of English and three non-native speakers from a Mandarin background, and the judgments and ratings of native English (American) speakers. Results indicate that the vowel space, as determined by properties of the three corner vowels [a], [i], and [u], correlates more strongly with the barrier task than any of the other measures used in this study. This finding is especially relevant to those in the field of English language assessment like ESL teachers and speech-language pathologists, who are interested in combining acoustic measures of language performance with deeper comprehension based measures.