This study aimed to examine the relationship between Vietnamese university students' learning experience and their vocabulary learning strategy use frequency and perceived usefulness in English as medium of instruction (EMI) classes. The research involved 152 expert EMI students and 183 novice EMI students. A methodological triangulation was used to collect the data, included Schmitt's (1997) VLS questionnaire, students' diaries and focus group interviews. A significant difference was found between novice and expert EMI students in their VLS use frequency, but no significant distinction was found between these two groups of EMI students in their VLS perceived usefulness. More specifically, the results indicated that novice EMI students used VLS more frequently than their seniors, but the expert EMI students perceived VLS usefulness as much as their juniors. Based on the findings, some pedagogical implications are put forwards in order to improve the teaching and learning of vocabulary for EMI students in Vietnam.