This study examines how a real-world task influences incidental vocabulary learning in Korean high school EFL students and identifies factors affecting this process. Twenty-eight male students, grouped in pairs, created travel-guide videos about a tourist site. To measure vocabulary learning and retention, students completed a pre-test, an immediate post-test, and a delayed post-test, assessing both receptive and productive knowledge. Vocabulary learning was calculated as the percentage of unfamiliar words correctly answered on each post-test. Results showed that immediate receptive learning was 24%, delayed receptive learning was 25.6%, immediate productive learning was 17.8%, and delayed productive learning was 17.1%. The study also found that prior vocabulary knowledge (PVK) and word frequency significantly impacted learning outcomes: students with higher PVK learned approximately four times more than those with lower PVK, and words encountered multiple times were learned twice as effectively as single-exposure words. Additionally, students provided valuable feedback on their vocabulary learning, offering practical insights for future curriculum design.