This study was set to cross-validate a bilingual Persian-English version of the Vocabulary Size Test (VST) against the monolingual English version and compare Iranian EFL learners' performance on the two versions. Various bilingual versions of the VST have been developed based on the assumption that bilingual versions are not affected by the grammar and reading demands of the long options in the monolingual version. To serve the purposes of the study, the Persian-English version and monolingual version of the VST were administered to 116 Iranian EFL learners. Results indicated that a single dimension was underlying both versions. Combined factor analysis indicated that both versions assessed the same construct. Further, both versions were capable of distinguishing learners of varying English proficiency levels and there was also a rough order of difficulty across frequency levels in both of them. Separate paired-samples t-tests revealed that the low- and mid-proficiency groups had differential performance on the two versions of the VST. Their L2 vocabulary knowledge was significantly underestimated by the monolingual version as shown by the mean-differences between the two versions of the VST for these two groups. In contrast, the results also revealed that the high-proficiency group in the study did not show such differential performance on the two version of the VST as the mean-difference between the two versions of the VST did not reach statistical significance for this last group. Hence, it is argued that advanced learners are competent enough so that their performance is not affected by grammar and reading demands of the long options in the monolingual version.