Vehicular networks require a reliable and efficient one-hop broadcast service to disseminate delay sensitive messages for high-priority safety applications. However, frequent link breakage in the dynamic networking conditions, in the presence of high relative mobility and channel fading, poses technical challenges in satisfying the strict service requirements. Makeup transmissions can improve the performance through repetitive broadcasting of packets before detecting a transmission failure. In this paper, we introduce a node cooperation based makeup strategy for vehicular networks, referred to as cooperative relay broadcasting (CRB), such that neighboring nodes rebroadcast the packet from a source node, increasing the reliability of the broadcast service. The decision to perform CRB is taken proactively and based on the channel conditions between the relaying nodes and the target one-hop neighbors. We propose an optimization framework that provides an upper bound on the CRB performance with accurate channel information. Further, we propose a channel prediction scheme based on a two-state first-order Markov chain to select the best relaying node for CRB. We study the reliability of the broadcast service in terms of packet received rate and packet delivery rate. Through extensive simulations, we demonstrate that the proposed CRB scheme provides a more reliable broadcast service, as compared with existing approaches.