One key aspect of the Internet of Things (IoT) is, that human machine interfaces are disentangled from the physicality of the devices. This provides designers with more freedom, but also may lead to more abstract interfaces, as they lack the natural context created by the presence of the machine. Mixed Reality (MR) on the other hand, is a key technology that enables designers to create user interfaces anywhere, either linked to a physical context (augmented reality, AR) or embedded in a virtual context (virtual reality, VR). Especially today, designing MR interfaces is a challenge, as there is not yet a common design language nor a set of standard functionalities or patterns. In addition to that, neither customers nor future users have substantial experiences in using MR interfaces. Prototypes can contribute to overcome this gap, as they continuously provide user experiences of increasing realism along the design process. We present ExProtoVAR, a tool that supports quick and lightweight prototyping of MR interfaces for IoT using VR technology.