Smartwatches and fitness trackers integrate different sensors from inertial measurement units to heart rate sensors in a very compact and affordable form factor. This makes them interesting and relevant research tools. One potential application domain is virtual reality, e.g., for health related applications such as exergames or simulation approaches. However, commercial devices complicate and limit the collection of raw and real-time data, suffer from privacy issues and are not tailored to using them with VR tracking systems. We address these issues with an open source design to facilitate the construction of VR-enabled wearables for conducting scientific experiments. Our work is motivated by research in mixed realities in pervasive computing environments. We introduce our system and present a proof-of-concept study with 17 participants. Our results show that the wearable reliably measures high-quality data comparable to commercially available fitness trackers and that it does not impede movements or interfere with VR tracking.