Data-driven learning (DDL) is an increasingly popular area of research for language teaching and is gradually being incorporated into teacher education programs globally. However, assessing teacher trainees' (self-reported) corpus literacy, and its impact on eventually (and successfully) incorporating corpora into language teaching, is an ongoing issue. This paper explores the experiences of in-service English language teacher trainees in developing both corpus literacy and corpus-based language pedagogy skills in Vietnam. The study finds a disconnect between participants' high self-reported corpus literacy skills, high intention to conduct corpus-based language teaching, and their ability to convert said intention and literacy into viable ideas for corpus-based language teaching within their lesson planning. Potential reasons include difficulties conceptualizing DDL's integration into lesson objectives, selection of inappropriate corpora for the target audience, issues planning DDL activities for primary-age learners, and Vietnamese-specific issues mainly around access to technology. Accordingly, we present several recommendations for improving the assessment of DDL trainees' actual integration of DDL into their teaching practice.