Few studies have estimated the frequency of e-cigarette use by smoking status among French young adults, and how those who smoke tobacco start and continue to use e-cigarettes. Our aim was to describe e-cigarette use among students who smoked tobacco. A multi-stage, mixed methods study was conducted at the University of Bordeaux between September 2018 and March 2020. The study consisted of three different sub-studies: 1) a cross-sectional study across five campuses (n = 211), 2) a qualitative study (n = 30), and 3) an online cross-sectional study (n = 415). These were combined to form an explanatory sequential design (stage 1) and then a convergent parallel design (stage 2). Although 41 % of students had tried e-cigarettes at least once in their lifetime, only 7 % were current users. Both e-cigarette experimentation and current use (i.e. occasional or daily use) were mainly found among current and former smokers. Student smokers started using e-cigarettes out of curiosity, with other vapers. Two main factors were identified as intervening in the transition from smoking to sustained vaping: the perception of smoking as problematic and personal commitment to e-cigarette use (i.e. by buying their own device, acquiring technical skills, and increasing the frequency of use). Among current vapers, exclusive vapers and dual users differed in terms of their smoking goals, the role they attributed to e-cigarettes, their identity development and their perceived social or personal benefits. This study highlighted the complexity of the decision-making process for transitioning from smoking to sustained vaping among university students. This required a socially supportive environment and some intrinsic factors, of which the problematization of smoking and personal commitment to vaping were key factors.