Research on the relationship between vocabulary knowledge and reading has largely focused on literal comprehension of informational texts. The present study, however, investigates the vocabulary-reading relationship by specifically examining critical reading skills in an argumentative text.76 EFL university students majoring in Education and studying in English participated in this study. To assess vocabulary knowledge, two tests were administered: a vocabulary size test (VLT) and a lexical coverage test. The coverage test was prepared by generating word families from the text using Vocabprofiler computer program software. Students' critical reading skills were assessed using a multiple-choice test format. The findings indicate that participants had a vocabulary size of 4,000 to 5,000 words. Both vocabulary size and text coverage correlated significantly with critical reading. However, this correlation does not apply for all the targeted critical reading skills. The study concludes that a minimum vocabulary size of 4,000 words would ensure approximately 97% of text coverage, leading to successful performance in critical reading. Below this threshold, participants' performance declined. Both vocabulary measures correlated with critical reading skills that required some reliance on the vocabulary of the text. The study suggests some pedagogical implications.