Validated under a Rasch framework (Beglar, 2010), the Vocabulary Size Test (VST) (Nation & Beglar, 2007) is an increasingly popular measure of decontextualized written receptive vocabulary size in the field of second language acquisition. However, although the validation indicates that the test has high internal reliability, still unaddressed is the possibility that it overestimates learner vocabulary size due to guessing effects inherent in its multiple-choice format, as size estimates are made by multiplying its raw score by a constant (100 or 200). This article argues that the VST's multiple-choice format results in a test of passive recognition of words that does not approximate the experience of readers of authentic English texts, details drawbacks of the Rasch framework and mean-square fit statistics in detecting the overall contribution of guessing effects to raw test scores that could have allowed such deficiencies to remain undetected during the test's validation, overviews challenges that multiple-choice formats pose for vocabulary tests, and concludes by proposing methods of testing and analysis that can address these concerns.