This article describes the development and validation of a Vietnamese bilingual version of the Vocabulary Size Test--a test which measures written receptive vocabulary size. The test can be used to measure the English vocabulary size of Vietnamese learners of English. A learner's total vocabulary size is calculated by multiplying their test result by 100. The research adds to our knowledge of vocabulary size testing in the following ways. First, it shows that a bilingual version of a monolingual test performs in much the same way as the monolingual test, distinguishing learners of different proficiency levels and returning lower scores at later levels of the test. Second, it shows that every level of the test should be sat, otherwise there will be a considerable underestimation of learners' vocabulary sizes. This suggests limitations to the previously accepted assumption that learners' vocabulary growth can be largely related to word frequency. Third, it shows that bilingual tests are feasible alternatives to more challenging and time-consuming monolingual tests. (Contains 8 tables.)