1. 1. Key Difference in Functionality: Docker Secrets is a built-in feature of Docker that allows for securely storing and managing sensitive information, such as passwords or API keys, to be used within Dockerized applications. On the other hand, Vault is a separate tool developed by HashiCorp that provides a centralized and highly secure way to manage secrets across different platforms and services.
2. 2. Integration with Orchestration Tools: Docker Secrets is tightly integrated with Docker Swarm, the built-in container orchestration tool, allowing secrets to be automatically distributed to services running on the Swarm cluster. Vault, on the other hand, can be integrated with any orchestration tool or platform, including Kubernetes, Docker Swarm, and Mesos.
3. 3. Granularity and Access Control: Docker Secrets provides basic access control by limiting access to secrets based on the principle of least privilege, allowing only authorized services to access specific secrets. In contrast, Vault offers much more advanced access control mechanisms, including fine-grained policies and roles, enabling more precise control over who can access specific secrets and under which conditions.
4. 4. Secret Encryption and Storage: Docker Secrets encrypts secrets using AES-256-GCM encryption and stores them securely on the Docker Swarm manager nodes. Vault, on the other hand, supports a wide range of encryption algorithms and storage backends, and it can be configured to encrypt secrets using hardware security modules (HSMs) for even higher security.
5. 5. Secret Rotation and Revocation: Docker Secrets does not have built-in support for secret rotation or revocation. Once a secret is created and distributed to services, it cannot be easily updated or revoked without restarting the affected services. In contrast, Vault has built-in support for secret rotation, allowing secrets to be easily rotated without affecting the services using them, and it also provides revocation mechanisms to immediately revoke access to specific secrets.
6. 6. Auditing and Compliance: Docker Secrets does not provide built-in auditing or compliance functionalities. It does not track who accessed or modified secrets or provide any logs or reports. In contrast, Vault offers comprehensive auditing capabilities, including logging of all secret access and modification events, as well as compliance features such as audit trail integrity protection and tamper-evident logs.

In Summary, Docker Secrets is a simple and straightforward built-in secret management feature of Docker Swarm, offering basic access control and encryption. On the other hand, Vault is a separate and more sophisticated secrets management tool with advanced access control, encryption, rotation, revocation, auditing, and compliance functionalities.