Question:

How to configure the Issuer for Single Sign-On (SSO) in Vault with Active Directory Federation Services (AFDS)?

Answer:

The issuer is the identifier of the relying party which also has to be configured in Active Directory Federation Services (AFDS). Generate and use an identifier which describes the Vault domain to be used in this setup.

Related Documentation:

KB Article: How to check the Relying party identifier (RPID) that defined in Active Directory Federation Services (ADFS) side 

Thank you