Adult second-language (L2) learners' perception of L2 phonetic segments is influenced by first-language phonological and phonetic properties. It was recently proposed that L2 vocabulary size in adult learners is related to changes in L2 perception (perceptual assimilation model), analogous to the emergence of first-language phonological function (i.e., attunement to the phonological identity of words) associated with the "vocabulary explosion" at 18 months. In a preliminary investigation of the relationship between L2 perception and vocabulary size, Japanese learners of Australian English identified Australian English vowels, provided goodness of fit ratings, and completed a vocabulary size questionnaire. We adopted a "whole-system" approach, allowing learners to apply all native vowel system possibilities to the full L2 vowel system. Learners with a larger L2 vocabulary were more consistent in their vowel assimilation patterns, compatible with the L2 perceptual assimilation model. (Contains 4 tables and 1 figure.)