In this paper, we propose a novel protocol called VC-MAC that utilizes the concept of cooperative communication tailored for vehicular networks, especially for gateway downloading scenarios. VC-MAC leverages the broadcast nature of the wireless medium to maximize the system throughput. The spatial and user diversity are exploited by the concurrent cooperative relaying to overcome the unreliability of the wireless channel in vehicular networks. We theoretically analyze the selection of optimal relay set using weighted independent set (WIS) model, and then design a back-off mechanism to select the concurrent relays in a distributed manner. Extensive simulations in ns-2 are carried out to demonstrate that compared with existing strategies, VC-MAC effectively enhances cooperative information downloading and significantly increases the system throughput.