The commercial adaptation of Vehicular Ad hoc NETwork (VANET) to achieve secure Intelligent Transportation System (ITS) heavily depends on the security guarantees for the end-users and consumers. Current VANET security standards address most of the security challenges faced by the vehicular networks. However, with the emergence of 5th Generation (5G) networks, and the demand for a range of new applications and services through vehicular networks, it is imperative to integrate 5G and vehicular networks. To achieve a seamless integration, various design and implementation issues related to 5G and VANETs must be addressed. We focus on the security issues that need to be considered in order to enable the secure integration of 5G and VANETs. More precisely, we conduct in-depth study of the current security issues, solutions, and standards used in vehicular networks and then we identify the security gaps in the existing VANET security solutions. We investigate the security features of 5G networks and discuss how they can be leveraged in vehicular networks to enable a seamless and efficient integration. We also propose a security architecture for vehicular networks wherein the current VANET security standards and 5G security features coexist to support secure VANET applications. Finally, we discuss some future challenges and research directions for 5G-enabled secure vehicular networks.