Overview:
A user trying to login to Vault is receiving an error: User with Vault Username [] was not found.
Root Cause:
The Vault SAML Profile is setup to use Vault usernames as the user identifier between the Identity Provider and Vault. The value within the brackets of this error message is not a valid Vault username in the Vault.
Solution:
Be sure that the uid attribute in the SAML Response is passing the correct Vault username for the user signing into this Vault.
Related Documentation:
Vault Documentation: About SAML User ID Types