It cannot be denied that vocabulary is considered to be the beginning of learning a language and determines the success of learners. For this reason, teaching and learning vocabulary need special attention from both teachers and learners. However, the design of vocabulary exercises in the English textbooks used in a school program in Viet Nam is little, which is a small part of the vocabulary designed in the Language section and a review exercise in Looking Back one. The study is aimed to evaluate the effectiveness of using supplementary vocabulary exercises to enhance vocabulary for 10th graders and collect students' feedback on the benefits and drawbacks of the supplementary vocabulary exercises. Thence solutions are proposed to enhance the effectiveness of the supplementary vocabulary exercises. The study was carried out using a quasi-experimental research design and survey. The study's participants were 41 students of which 20 students were selected randomly to the experimental group (using a set of supplementary vocabulary exercises) while 21 of them were assigned to the control group (non-using supplementary vocabulary exercises). While all 41 students were asked to participate in pre-tests and post-tests, only those in the experimental group responded to questions about the difficulties when using supplementary exercises and their desire to use them. Following the use of supplementary exercises, the finding reveals that the students' vocabulary was improved significantly after 15 weeks of the experiment. Furthermore, students' feedback on the use of supplementary exercises is generally active, and recommendations for using a set of supplementary exercises are offered in order to make the supplementary exercises more relevant and efficient.