Purpose: This study examined the efficacy of the Vocabulary Acquisition and Usage for Late Talkers (VAULT) treatment in a version that manipulated the length of clinician utterance in which a target word was presented (dose length). The study also explored ways to characterize treatment responders versus nonresponders. Method: Nineteen primarily English-speaking late-talking toddlers (aged 24-34 months at treatment onset) received VAULT and were quasirandomly assigned to have target words presented in grammatical utterances matching one of two lengths: brief (four words or fewer) or extended (five words or more). Children were measured on their pre- and posttreatment production of (a) target and control words specific to treatment and (b) words not specific to treatment. Classification and Regression Tree (CART) analysis was used to classify responders versus nonresponders. Results: VAULT was successful as a whole (i.e., treatment effect sizes of greater than 0), with no difference between the brief and extended conditions. Despite the overall significant treatment effect, the treatment was not successful for all participants. CART results (using participants from the current study and a previous iteration of VAULT) provided a dual-node decision tree for classifying treatment responders versus nonresponders. Conclusions: The input-based VAULT treatment protocol is efficacious and offers some flexibility in terms of utterance length. When VAULT works, it works well. The CART decision tree uses pretreatment vocabulary levels and performance in the first two treatment sessions to provide clinicians with promising guidelines for who is likely to be a nonresponder and thus might need a modified treatment plan.