[armon]

When Vault connects to the endpoint system to create a dynamic user, it presents a set of credentials only known to it. You have to authorize Vault to create dynamic users, so a malicious actor would need to somehow obtain a similar level of privilege.

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.