As integrated writing tasks in large-scale and classroom-based writing assessments have risen in popularity, research studies have increasingly concentrated on providing validity evidence. Given the fact that most of these studies focus on adult second language learners rather than younger ones, this study examined the relationship between written discourse features, vocabulary support, and integrated listening-to-write scores for adolescent English learners. The participants of this study consisted of 198 Taiwanese high school students who completed two integrated listening-to-write tasks. Prior to each writing task, a list of key vocabulary was provided to aid the students' comprehension of the listening passage. Their written products were coded and analyzed for measures of discourse features and vocabulary use, including complexity, accuracy, fluency, organization, vocabulary use ratio, and vocabulary use accuracy. We then adopted descriptive statistics and hierarchical linear regression analyses to investigate the extent to which such measures were predictive of integrated listening-to-write test scores. The results showed that fluency, organization, grammatical accuracy, and vocabulary use accuracy were significant predictors of the writing test scores. Moreover, the results revealed that providing vocabulary support may not necessarily jeopardize the validity of integrated listening-to-write tasks. The implications for research and test development were also discussed.