Comment 1 for bug 1845933

The Vault charm defines a peer relation in its metadata.yaml but never uses it. Presumably, this was either an oversight for put in place for some future use-case that has not been implemented.

The "Unable to find implementation for relation" message is just warning that the Vault charm author may have forgotten to add 'interface:vault-ha' to the layer.yaml's includes section and that the charm may be expecting functionality provided by that layer which isn't available. However, since as far as I can tell, the vault-ha interface layer simply does not exist, and there is no code in the Vault charm which would use it even if it did, this message is just spurious.

I think it is, in fact, a red herring for the real issue you're encountering. Is there any chance you could provide a more complete set of debugging logs, such as via juju-crashdump (snap install juju-crashdump)?