This paper examines the potential reasons of interlingual errors committed by Vietnamese learners of English (Vietnamese English) from the paper-based survey on forty Tra Vinh University students in Vietnam by comparing two languages’ differences in grammar and how the errors have possibly occurred owing to the Vietnamese interference onto English. The error analysis focuses upon four categories: inflectional morphosyntax, article, word order, and copula ‘to be’. The findings have indicated that in inflectional morphosyntax, the errors were found in tense and aspect (100%), subject-verb agreement (100%) and noun plural inflection (97.5%), where the suffixal morphemes ‘-s’ and the tense and aspect features were omitted. In a similar way, copulas ‘to be’ were omitted before adjective with 30%. The next category is article whose errors (100%) occurred owing to confusion. Finally, the errors of three subcategories in word order were found including noun modifier position within a noun phrase (100%), adjective position within a noun phrase (97.5%), and adverb position within a verb phrase (100%). This research study provides not only Vietnamese but also foreign teachers and students of English with practical problems of grammar use that might help them seek out effective way in teaching and learning process.