Vocabulary plays a significant role in the learning process of English as a Foreign Language (EFL) students, especially undergraduates who need to read academic materials and listen to specialized lectures in English. However, retaining lexical items is still a challenging task for many EFL learners. The current study seeks to examine the effectiveness of a constructivist approach in improving EFL learners' vocabulary retention, mainly meaning recall, as well as investigate their attitude towards this learning method. To achieve this aim, a quasi-experimental design, including a pretest and a posttest, was employed with the participation of 75 EFL Vietnamese undergraduates. The participants were assigned into two groups: the control group (Group 1, N = 37) and the experimental group (Group 2, N = 38). While Group 1 was taught in a conventional way, Group 2 received vocabulary training following the constructivist approach. At the end of the eight-week treatment, a questionnaire was administered to the experimental group to gauge their view on vocabulary learning using constructivism. The results showed that Group 2 retained lexical items more effectively than Group 1. Furthermore, the experimental group had a favorable opinion of learning vocabulary in the constructivist approach. Recommendations for future studies as well as implications for teachers regarding vocabulary and constructivism, were presented in detail.