In vehicular ad hoc networks (VANETs), authentication and key agreement (AKA) protocols are crucial for establishing secure authentication and session keys for network communications, ensuring security for short-term vehicle interactions. However, traditional VANETs AKA schemes heavily rely on centralized trust architectures. This dependence raises significant concerns about overall system security and resilience, especially when these schemes operate over insecure wireless channels. Although recent studies have introduced decentralized architectures to mitigate this issue, a key limitation remains. These approaches typically assume synchronous network environments, which in practice limits their true decentralization capabilities in dynamic VANETs. To address these challenges, we propose a decentralized and asynchronous AKA scheme based on a consortium blockchain that enables the execution of the key agreement process in asynchronous VANETs environments. Furthermore, we employ lightweight cryptographic techniques combined with a Cuckoo filter to optimize computational efficiency, reduce communication overhead, and minimize on-chain storage costs. Security analyses and simulation experiments demonstrate that the proposed scheme delivers robust security and high performance under realistic network conditions.