This study examines how different task types impact lexical richness in Vietnamese EFL students’ writing. It focuses on lexical diversity, density, and sophistication in opinion and compare/contrast essays. Results show significant differences in these lexical aspects between essay types. Opinion essays exhibit higher lexical diversity and density, while Compare/contrast essays use more academic vocabulary, indicating distinct lexical sophistication levels.This study examines how different task types impact lexical richness in Vietnamese EFL students’ writing. It focuses on lexical diversity, density, and sophistication in opinion and compare/contrast essays. Results show significant differences in these lexical aspects between essay types. Opinion essays exhibit higher lexical diversity and density, while Compare/contrast essays use more academic vocabulary, indicating distinct lexical sophistication levels.