E-cigarette use in the United States has significantly grown in recent years. Widespread diffusion of e-cigarette content across social media communities may be contributing to this growth. In this study, we (1) explored topics related to e-cigarettes and vaping on Reddit and (2) examined the extent to which these topics clustered across distinct communities. We analyzed a total of N = 79,783 Reddit submissions posted between March 2017 and February 2018 that mentioned at least one e-cigarette or vaping keyword. We created a dictionary to classify submissions into seven different topics related to e-cigarettes and vaping. Submissions were also categorized into one of six mutually exclusive communities identified using subreddit meta-data. Our results indicate that e-cigarette and vaping content on Reddit is primarily about the buying and selling of e-cigarette products. Other common topics included how to build vaping devices, e-juice, and e-cigarette advice. Network correlation analyses found that the distribution of our seven identified topics varied significantly among general e-cigarette, drugs, and research/news subreddit communities. Findings from this study add to a growing literature investigating e-cigarettes and vaping on social media and also contribute to network-level theories by linking communities on Reddit to the diffusion of various depictions of e-cigarettes and vaping.