While in recent years, the introduction of English as a Medium of Instruction (EMI) courses has been increasing in Taiwanese and Vietnamese universities, use of EMI courses is still in an early phase, and their effectiveness remains under-explored in both territories. This study aims to develop an EMI effectiveness scale and apply importance-performance analysis (IPA) to measure student assessment of EMI courses based on that scale. Using the convenience sampling method with structured questionnaires to collect the data, 815 usable samples were collected, representing 490 Taiwanese and 325 Vietnamese undergraduate students. The findings describe successful development of an EMI effectiveness scale comprised of three factors with 19 items. IPA analysis determined that, in both territories, the most important EMI factor considered was the teachers' teaching characteristics, while 'cross-cultural and international outlook development' was perceived as the most underperformed and least important dimension. The analysis also shows that different territories have various weaknesses and strengths in EMI; territories could benefit from studying other territories' outperforming areas. Suggestions for improving EMI in both territories' university contexts are discussed.