Background: Pragmatic communication difficulties encompass many distinct behaviours, including the use of vague and/or insufficient language, a common characteristic following traumatic brain injury (TBI) that negatively impacts psychosocial outcomes. Existing assessments evaluate pragmatic communication broadly, often with only one or two items capturing each behaviour, thus limiting sensitivity and precision to variations within each behaviour. Given that greater nuance is needed to detect subtle pragmatic communication differences and investigate underlying cognitive mechanisms, a more refined measure is critical to improve psychosocial outcomes. The Vague scale was developed to address those needs. Aim: To provide preliminary evidence supporting the novel Vague language use (Vague) scale's reliability, validity and clinical utility. Methods and Procedures: The Vague scale rates each discourse sample utterance for vague language use on a 3-point scale; the measure's Vague score represents the mean of utterance-level ratings. Using the Vague scale, two raters naïve to diagnosis evaluated Cinderella narratives of 46 adults with severe TBI and 46 controls with no brain injury, providing reliability, construct validity and classification accuracy evidence. Vague scores were also compared to other clinical measures to gather criterion-related validity evidence. Outcomes and Results: Interrater agreement across all transcripts was moderate. Construct validity was supported by expected group differences and criterion validation, including significant relationships with increased violations of Grice's maxim of quantity and measures of lexical variation; significant relationships with psychosocial outcomes, supporting clinical utility; and nonsignificant relationships with measures of syntax and overall pragmatic communication. Classification accuracy expectedly did not support using Vague scores in isolation for diagnosis, due to unacceptable sensitivity (0.696). Conclusions and Implications: Evidence supported the Vague scores' psychometric properties. Thus, the Vague scale shows promise as a measure of one distinct pragmatic communication behaviour: vague language use. Future research should apply the Vague scale to determine its sensitivity in individuals with subtle social communication challenges (e.g., mild TBI), explore its utility with more naturalistic discourse samples as part of a pragmatic communication battery, longitudinally examine changes in Vague scores, and investigate cognitive mechanisms underlying this specific pragmatic communication behaviour.