Competency in society's lingua franca plays a major role in the emergence of social disparities within education. Therefore, the present longitudinal study investigates vocabulary development and its predictors in preschool years. We focus on whether internal (phonological working memory) and external variables (preschool and home learning environment) have different impacts depending on parental native language. The study considers 547 children from 97 German preschools. Children's vocabulary was assessed at the ages of 3, 4, and 5 years. Latent growth curve models show that non-native German language children are characterized by reduced vocabulary at first assessment and lower progress compared to monolingual peers. Phonological working memory has a strong impact on all children's initial vocabulary but also on vocabulary growth in those whose parents speak German as an additional language. The effects of preschool and home learning environment are comparatively smaller. (author abstract)