Effective use of vocabulary contributes to academic language proficiency and academic success (Douglas, 2013). Sophisticated vocabulary use relates to increased quality of writing (Laufer & Nation, 1995; Kyle & Crossley, 2015). However, it is unclear which lexical sophistication characteristics contribute to writing quality assessments. Furthermore, previous studies focused on general assessments, rather than English for Specific Purposes diagnostic assessments which aid in early intervention for academic support. The present study investigates the relationship between vocabulary sophistication indices and writing scores (N = 353) on a post-entry university diagnostic test for engineers (Fox & Artemeva, 2017). Multiple lexical sophistication approaches were compared to writing scores and differences in lexical profile characteristics of successful and unsuccessful students were compared. Results indicated that samples of successful writing have a higher presence of tokens, types, lexical stretch, academic vocabulary and formulaic language. The findings have pedagogical implications for remedial writing instruction of engineering students.