Purpose: To differentiate the effect of phonotactic probability from that of neighborhood density on a vocabulary probe administered to preschool children with or without phonological delays. Method: Twenty preschool children with functional phonological delays and 34 preschool children with typical language development completed a 121-item vocabulary probe in both an expressive and receptive response format. Words on the vocabulary probe orthogonally varied on phonotactic probability and neighborhood density but were matched on age of acquisition, word frequency, word length, semantic set size, concreteness, familiarity, and imagability. Results: Results show a Phonotactic Probability x Neighborhood Density interaction with variation across groups. Specifically, the optimal conditions for typically developing children were rare phonotactic probability with sparse neighborhoods and common phonotactic probability with dense neighborhoods. In contrast, only rare phonotactic probability with sparse neighborhoods was optimal for children with phonological delays. Conclusions: Rare sound sequences and sparse neighborhoods may facilitate triggering of word learning for typically developing children and children with phonological delays. In contrast, common sound sequences and dense neighborhoods may facilitate configuration and engagement for typically developing children but not for children with phonological delays because of their weaker phonological and/or lexical representations.