The main objective of this study is to investigate how interactive factors affect the vocabulary usage of second language learners in their spoken language. Participants were 24 L1 Chinese undergraduate students of L2 English at an advanced level. L2 learners’ vocabulary use was assessed via tokens, lexical diversity, and frequency-based lexical sophistication. Participants provided speech data in response to seven persuasive speaking tasks across three speaking modes: two monologic, two dialogic, and three trialogic. This study showed that the interactive factor has a varied effect on L2 learners’ vocabulary usage. It positively influences the use of advanced vocabulary but does not affect the total number of words produced or the diversity of words used. Second, of all three speaking modes, the dialogic speaking mode is the best speaking condition to trigger L2 learners’ use of advanced words. Third, the vocabulary employed in dialogues and trialogues can vary due to the inherent disparities between the two modes of speech. Therefore, we propose the use of the dialogic interactive factor and trialogic interactive factor instead of the term “interactive factor” to encompass two specific conditions in which there was a noticeable difference in the performance of L2 learners.