Williams Syndrome (WS) children possess relatively large vocabularies when compared to their other, impaired, cognitive skills. However, their language acquisition is delayed, their vocabulary does not spurt, categorisation skills are weak and they do not respond taxonomically. We mimic WS categorization impairments by hindering the formation of visual categories in a model of early word learning (Mayor & Plunkett, 2010). In the absence of lesions, the model accounts for the emergence of taxonomic responding and displays a vocabulary spurt driven by the formation of categorical representations. In contrast, when categorisation is impaired, lexical acquisition is delayed, a vocabulary spurt is absent and word-object associations are not generalised. However, through repetitive labelling events, the system is still able to acquire a large "proto-lexicon" by gradually attaching several exemplars of a category to their appropriate sound pattern in an associationist mechanism, thereby leading to the formation of a surprisingly large vocabulary.