This study investigated the role of vocabulary size in assessing learners from various L1 backgrounds and for institutional placement. Participants included native speakers of Spanish, Korean, Japanese, and Mandarin, who took a test that estimated the size of their productive English vocabulary. First, the vocabulary size of students from each language group was compared and the relationship between vocabulary size and institutional placement level was examined. Then, scores were analyzed to determine how cognates and loan words influenced the participants' performance on the vocabulary test. Further, students' vocabulary size scores were correlated with their placement scores that evaluated reading, writing, speaking, listening, and grammar. Finally, the vocabulary size of new students was compared to returning students within the same institutional placement level. Within the same institutional placement levels, speakers of Spanish and Korean had larger English vocabularies than speakers of Japanese and Chinese. It was also found that there is a clear distinction between students at varying institutional placement levels in terms of their vocabulary size. The cognate analysis revealed that students learn cognates faster than non-cognates. The correlation analysis revealed that vocabulary size correlated most with speaking, and listening, followed by grammar, then reading and writing. Finally, the cross-sectional analysis indicated that the vocabulary size of newly placed students was generally larger than that of continuing students.