The purpose of this study is to investigate whether C-Tests, which have been tested with international ESL cohorts in New Zealand, can be used as reliable English proficiency measures in Vietnam. C-Tests produce robust reliability and validity in most SLA studies. The three C-Tests that have been used at Massey Univer-sity, New Zealand, were used as a basis of comparison for the Vietnamese sample, as they have been trialled and revised using classical item analysis, reliability studies and construct/concurrent/criterion validity checks against IELTS/TOEIC scores in New Zealand, thereby considered reliable and valid for various Asian and Middle Eastern ethnic groups. The findings of this study show that the three C-Tests have acceptable reliabil-ity and significant correlations among themselves, and can be used to evaluate overall English proficiency in Vietnam. The results administered to the Vietnamese cohort in this study are expected to be added to the growing number of other ethnic groups for which they evaluate overall English proficiency validly, reliably, and efficiently. There are implications discussed here for test developers working with C-Tests in making them reliable and valid measures of English proficiency.