My concern is how you can tell me whether a database user was created by Vault and not by a malicious actor with knowledge of Vault's Dynamic Secrets method mimicking it.
How the heck can I know if it was a legit created database user or not?
How the heck can I know if it was a legit created database user or not?
Vault typically prefixes something to the username as well (e.g. "vault-...") and also audits the creation of dynamic users so you can either look for the prefix or cross check the audit logs.
But based on what you've just asked, I'd definitely never create anything like what you're working with because it sounds like ANY addition to your infrastructure is problematic.
So whatever is accepting the secret must be checking with Vault to verify the secret.