I recently gave a talk at the Kafka WU summit 2021 about how to encrypt Kafka messages at rest for inceased application security. One of the questions that cam out of the session was about batch encryption / decryption of multiple messages.
I can confirm that the Vault API does allow batch encryption / decryption of multiple messages. For more information and example payloads, please refer to the documentation here.
This allows for more flexible workflows to suit various application architectures.