AWS Backup: Regarding the AWS backup for RDS to logically air-gapped vault.

According to the available information, AWS Backup's logically air-gapped vault feature already supports Amazon RDS backups. The preview announcement from August 9, 2023 specifically mentions that the logically air-gapped vault supports backup and restore of Amazon Relational Database Service (RDS) along with other services like Amazon EC2, Amazon EBS, Amazon S3, and Amazon EFS.

The logically air-gapped vault is designed to allow secure sharing of backups across accounts and organizations, with direct restore capabilities to help reduce recovery time from data loss events. You can target backups to a logically air-gapped vault by specifying it as a copy destination in your backup plan, and the vault can be shared for recovery with other accounts using AWS Resource Access Manager (RAM).

If you're experiencing issues with RDS backups in your logically air-gapped vault setup, it might be due to specific configurations or limitations not mentioned in the documentation, or possibly because the feature was initially in preview. Since the feature was announced in preview mode in the US East (Virginia) Region, you might want to check if there are any regional limitations or if the feature has fully rolled out to all regions.
Sources
Announcing AWS Backup logically air-gapped vault (Preview)

This is what i mean by copying to the Logically air-gapped vault not supporting RDS, all the details in this table. https://docs.aws.amazon.com/aws-backup/latest/devguide/backup-feature-availability.html#features-by-resource:~:text=Amazon%20RDS%20single%20instance

Yes, currently, RDS Backups are not supported in LAG Vault. There is already feature request in place with internal Service teams.

You can engage your AWS Backup Support team or Account Teams for additional details on the tentative availability.