Research to increase the early vocabulary development of urban students has emphasized the central role of teachers and the ways in which teachers use intervention curricula and strategies in their classroom contexts. This study explores teachers' fidelity to different components of a vocabulary intervention, specifically their use of prescribed procedural routines and language strategies as well as teachers' application of language strategies beyond what was scripted by the program, and the association between these different forms of intervention use and curriculum-specific vocabulary. Teacher fidelity to the intervention was explored across thirteen urban kindergarten classrooms. Measures of procedural and process-oriented fidelity were used to capture teachers' adherence to a vocabulary intervention, and transcripts of teacher language use during intervention observations were analyzed and coded to capture teachers' use of language extensions, language strategies that went beyond the materials or scripts provided by the intervention. Employing multi-level modeling results indicate that teachers' use of language extensions positively predicted student curriculum vocabulary performance after the 24 weeks of the intervention above and beyond surface forms of fidelity. Findings from the current study encourage the need for teacher training on evidence-based practices that focus not only on intervention procedures, but also on how to use program features and strategies to more flexibly address the needs of students and anticipate how to intensify or broaden the scope of instruction based on the classroom context.