Use of e-cigarettes (vaping) has potential to help pregnant women stop smoking. This study explored factors influencing adherence among participants in the vaping arm of the first trial of vaping for smoking cessation in pregnancy. We conducted semi-structured telephone interviews (n = 28) with women at three-months postpartum. Interviews were analysed using thematic analysis, informed by the Theoretical-Domains Framework, Necessity-Concerns Framework and Perceptions and Practicalities Approach. Interviewees generally reported high levels of vaping. We found that: (1) intervention adherence was driven by four necessity beliefs—stopping smoking for the baby, and vaping for harm reduction, smoking cessation or as a last resort; (2) necessity beliefs outweighed vaping concerns, such as dependence and safety; (3) adherence was linked to four practicalities themes, acting as barriers and facilitators to vaping—device and e-liquid perceptions, resources and support, whether vaping became habitual, and social and environmental factors; and (4) intentional non-adherence was rare; unintentional non-adherence was due to device failures, forgetting to vape, and personal circumstances and stress. Pregnant smokers provided with e-cigarettes, and with generally high levels of vaping, had positive beliefs about the necessity of vaping for smoking cessation which outweighed concerns about vaping. Non-adherence was mainly due to unintentional factors.