The article presents an empirical study that investigates the single- and cross-modality relationships between different dimensions of receptive vocabulary knowledge and language skills, as well as the importance of academic vocabulary knowledge in academic listening and reading comprehension. An Updated Vocabulary Levels Test (UVLT), a Vietnamese version of the Listening Vocabulary Levels Test (LVLT), an International English Language Testing System (IELTS) listening test and an academic IELTS reading test were administered to 234 tertiary level Vietnamese learners of English as a foreign language (EFL). Research findings showed that (1) orthographic and aural vocabulary knowledge were strongly correlated (r = 0.88) and of equal significance to L2 listening and reading comprehension, (2) receptive vocabulary knowledge was a very powerful and reliable predictor of learners' receptive language proficiency, (3) knowledge of academic vocabulary strongly correlated with academic listening (r = 0.65) and reading (r = 0.60) comprehension and the mastery of the Academic Word List (AWL) could suggest a band score 6.0 in both the IELTS listening and academic reading tests.