Background The vagus nerve is a neural nexus between the brain and body, enabling bidirectional regulation of physiological and affective processes. Contemporary models conceptualise emotion as a dynamic brain–body regulatory process arising from bodily state, appraisal and contextual meaning. Dysregulated arousal and affective valence may contribute to persistent emotional distress and increased vulnerability to emotional disorders. Interventions targeting vagal-autonomic pathways, including auricular vagal neuromodulation and breathing-supported heart-rate variability biofeedback, may therefore provide a mechanistically grounded approach to strengthening affective self-regulation. However, their independent and combined effects remain insufficiently characterised within controlled factorial designs. Objective This article situates LasaiON within the theoretical and empirical framework of brain–body regulation and describes its intervention protocol. Grounded in contemporary models of emotion that integrate the brain–body axis, LasaiON is conceived as a multimodal self-regulation programme combining transauricular vagal neuromodulation with slow-breathing supported heart-rate variability biofeedback to engage complementary vagal–autonomic pathways. The randomized, sham-controlled factorial protocol is designed to examine the independent and combined effects of these components and to determine whether this dual-access strategy can strengthen affective self-regulation and modulate early psychophysiological markers of emotional experience. Methods We will use a randomized, sham-controlled 2 × 2 factorial design to examine the effects of active versus sham transauricular vagal neuromodulation therapy and slow-breathing heart-rate variability biofeedback versus a natural-breathing control condition in healthy adult volunteers. Participants (n = 200) will be assigned to one of four experimental arms and will complete five consecutive 60-min laboratory sessions. Baseline and post-intervention assessments will include psychometric indices of anxiety, depressive symptoms and well-being, alongside heart-rate variability, electrodermal activity, exploratory eight-channel EEG and verbal–cognitive measures of affective experience. The design is intended to evaluate the independent, combined and short-term cumulative effects of exogenous auricular vagal neuromodulation and endogenous respiratory–cardiovagal training on early markers of emotional distress regulation. Discussion Demonstrating that transauricular vagus nerve stimulation and slow-breathing supported heart rate variability-biofeedback modulate early psychophysiological markers of emotional distress regulation would support the role of vagal-autonomic pathways in affective self-regulation. By testing active versus sham auricular vagal neuromodulation together with active versus control respiratory–cardiovagal training, LasaiON may clarify whether these approaches act independently, additively or synergistically. This would provide a mechanistic basis for future brain–body interventions designed to strengthen emotional regulation through combined bioelectronic and behavioural methods. Clinical trial registration Trial registration: ClinicalTrials.gov, NCT07498881. Registered 23 March 2026.