In this work, we present a framework analysis of millimeter waves (mmWaves) vehicular communications systems. Communications between vehicles take place through a cooperative relay which acts as an intermediary base station (BS). The relay is equipped with multiple transmit and receive antennas and it employs decode-and-forward (DF) to process the signal. Also, the relay applies maximal ratio combining (MRC), and maximal ratio transmission (MRT), respectively, to receive and forward the signal. As the vehicles' speeds are relatively high, the channel experiences a fast fading and this time variation is modeled following the Jakes' autocorrelation model. We also assume narrowband fading channel. Closed-form expressions of the reliability metrics such as the outage probability, the probability of error and the channel capacity are derived. Capitalizing on these performances, we derive the low and high power regimes for the capacity, and the high signal-to-interference-plus-noise-ratio (SINR) asymptotes for the outage and error probability to get full insights into the system gains such as the diversity and coding gains.