As connectivity of road traffic grows, so does the concern for its reliability and security. On the other hand, real-world testing of large-scale vehicular ad-hoc networks would be extremely costly. This paper presents a mixed-reality proof-of-concept for evaluating the communication of real vehicles using standard V2X messages while simulating the rest of the traffic. Detailed simulation of large-scale communication networks is also a computationally complex task. To be able to achieve the desired real-time simulation performance for mixed-reality testing, a so-called mesoscopic communication node was introduced, fusing the communication of the simulated vehicles into a single entity. This single entity realizes a sensor spoofing on other communicating vehicles. To verify the feasibility of the proposed approach, real-worlds tests were carried out involving two cars equipped with V2X devices and a third device transmitting standard messages from the simulation to the real vehicles in an “informed” denial-of-service-like way. Test results show that the mesoscopic simulation can retain real-time performance while replicating the communication of over 100 vehicles. Real vehicles can extract these virtual messages and packet drops remain in a plausible range.