This article aims to uncover how alignment affects second language (L2) vocabulary acquisition by Chinese-speaking learners of English as they interact with various interaction loads (i.e. input text, peers and video). It also explores how tasks with varying interactional intensity in relation to the interaction loads influence the alignment magnitude and vocabulary learning outcomes. To this end, two studies were conducted. In Study 1, three groups of L2 learners were instructed to learn and use the target words while performing paraphrase, summary or continuation task. The results indicated that task-induced interactional intensity had robust effects on alignment magnitude as well as receptive and productive lexical knowledge. In Study 2, three groups of L2 learners were asked to complete a continuation task while interacting with high-proficiency peers (HL group), low-proficiency peers (LL group) or with both low-proficiency peers & video (LLM group). The results showed that interacting with high-proficiency partners had positive impact on alignment magnitude and lexical knowledge, but video exerted only a limited effect. The findings afforded evidence for the 'interaction-alignment-learning' research route and accounted for vocabulary acquisition from a new perspective.