We study the cooperative secure transmissions in (multiple-input signal-output) MISO vehicular relay networks where the infrastructure node with multiple antennas sends two confidential messages to two legitimate vehicles with only one antenna, respectively. By exploiting the signal superposition and cooperative jamming techniques, the legitimate vehicle near the infrastructure node directly decodes its own messages and is also responsible for relaying messages for the other legitimate vehicle who is distant from the infrastructure node. To combat eavesdropping threat from a malicious node for these two legitimate vehicles, we first provide comprehensive analysis of the secrecy performances and then design optimal secure transmission schemes for both vehicles. In particular, we first derive the closed-form expressions of the transmission outage probability (TOP) and secrecy outage probability (SOP), which offer mathematic tools for separate control with respect to security and system throughput. Following these results, we develop the optimized transmission schemes for two typical secure communication scenarios. One scenario is to maximally suppress the message leak while maintaining satisfied transmission rate. To achieve this goal, we derive the optimal power allocation strategy to minimize the SOP under the upper bounded TOP requirement. The other scenario is to improve the average secrecy rate with system throughput taken into consideration. Corresponding, we propose a prioritized and an explicitly parameterized power allocation schemes, which address the mutual impact across legitimate vehicles transmissions by different approaches. Numerical results are presented to validate the derived theory analytical results and show the performance superiority of the proposed schemes in terms of the average secrecy rate and the SOP performance.