This quasi-experimental study investigated the short- and long-term effects of Intercultural Vocabulary Instruction on the argumentative writing performance of sixty undergraduate EFL students in Vietnam. Using a Pretest-Posttest-Delayed Posttest design, participants were divided into an experimental group (n = 30) receiving culturally contextualized lexical training, and a control group (n = 30) taught via conventional methods. Data analyzed through paired and independent t-tests showed no significant baseline differences. However, the experimental group significantly outperformed the control group in the immediate posttest (p < .001) and demonstrated superior retention in the delayed posttest (p < .001), while the control group suffered significant performance decay. The findings suggest that integrating cultural contexts anchors vocabulary in long-term memory and improves argumentation depth.