RSA-BT (Respiratory Sinus Arrhythmia biofeedback-based Breathing Training) is a common cardiorespiratory intervention that has been commonly used as a complementary treatment to diseases (e.g., asthma), and as an effective exercise to reduce anxiety. In this demo, we propose BreathCoach, a smart and unobtrusive system using sensors on smartwatch and smartphone-based VR that enables in-home RSA-BT coaching. Specifically, BreathCoach uses off-the-shelf devices to continuously monitor key bio-signals including breathing pattern (BP), inter-beat interval (IBI), and the amplitude of RSA, and intelligently calculates the optimal breathing pattern based on current and historical measurements. The recommended breathing pattern is then conveyed to the user in the form of an intuitive VR game to provide an immersive training experience. We will showcase a research prototype implemented on Android smartphone and smartwatch with two proof-of-concept VR game designs.