Supplementary exercises were fundamental and beneficial for students in that they provided essential support accompanying the regular teaching and learning program or textbook. In the era of technological development, supplementary exercises on online platforms allow flexibility, accommodate learners’ different needs and styles, and thus increase their learning quality. This study implemented online vocabulary exercises as supplementary tasks attached to the official English 10 textbook used in a Vietnamese high school (Global Success series). The study aimed to enhance students' vocabulary acquisition and proficiency through interactive online learning activities. The experimental method was employed to investigate the effect of online supplementary exercises on students' English vocabulary competence. The findings reveal the effectiveness of the designed online vocabulary exercises in facilitating vocabulary acquisition and retention among students. Through interactive and engaging activities, students demonstrated improved vocabulary comprehension, usage, and retention. Moreover, the exercises were found to enhance student motivation and engagement in the English 10 curriculum, fostering a positive learning environment conducive to language acquisition.