This study primarily examined the predictions of the strong version of the Contrastive Analysis Hypothesis (CAH) on Taiwanese vocational high school EFL students’ acquisition of two English vowels (/i/ and /ɪ/) by investigating the learners’ relative perceptual and productive difficulty of the vowels. To reach the goals, a listening identification test and a pronunciation test that were designed by the researchers were administered, and two research questions were formed. A total of 39 students from a private vocational high school in northern Taiwan were recruited to participate in this research. Their accuracy in perceiving and pronouncing the tested vowels was rated via the self-designed identification and production tests. After all the participants’ test scores were collected, two independent-samples t-tests were conducted to analyze the data. The statistical results of the study showed that the relative perceptual difficulty of the two vowels for the students was /i/ = /ɪ/ but the relative productive difficulty for the learners was /ɪ/ > /i/ (> means more difficult than). The findings suggested that the predictions of the strong version of the CAH only worked for the participants’ acquisition of the vowels in production.