As Halle & Marantz (2008: 71) acknowledge, “we have no real idea about how a child assigns features to Vocabulary Items” in Distributed Morphology (DM). Stated generally, how do children acquire language-specific (sometimes variable) mappings between morpho-syntactic features and their morpho-phonological exponents? Following Emonds (1986) in a DM framework, this article advances a testable ‘morphological transparency’ constraint on the acquisition of Vocabulary, and presents supporting results from a pilot observational child-language study in Danish. This constraint explains a significant difference in the mechanisms of Germanic case morphology. By hypothesis, ‘vestigial’ case forms of English and Danish pronouns are contextual allomorphs, with Vocabulary that do not contain any morpho-syntactic case features. Vestigial-case mechanisms constitute a comprehensive analysis of intra-individually variable case-form mismatches in coordinate Determiner Phrases, predicate nominals, and other syntactic structures. Thus, a principle of language acquisition ultimately explains the distribution of case forms both within and across language varieties.