With the extensive deployment of vehicular ad-hoc networks (VANETs), it becomes an inevitable choice to provide enhanced in-vehicle services for uploading a vast amount of shared vehicular data to cloud storage. However, there is still a lack of effective deduplication and audit methods for cloud-stored data in VANET scenarios. To address the securities of cloud-stored data in VANETs, we propose a blockchain-based data deduplication and distributed audit scheme for shared data under cloud-fog computing-based VANETs in this paper. In our scheme, we construct a distributed audit model for VANETs, where road side units (RSUs) are partitioned as multiple management areas. Each management area can solely make their consensus for data integrity verification to audit the cloud storage provider without depending on any third-party auditors (TPAs). Also, we establish a blockchain-based monitoring mechanism maintained by the fog servers to ensure the integrity of the uploading and auditing records and enable related entities within the system to verify corresponding audit results (or records). Furthermore, we propose a lightweight dual-verifier structure to adapt to resource-constrained VANET scenarios. Through our dual-verifier mechanism, our scheme can effectively resist proof-replay attacks. Related theoretical analysis and experimental results show our data deduplication and distributed audit scheme is efficient and effective for VANET scenarios.