A VSS credential is saved on both the VSS host and the NAS server. The server address and port number are saved along with the credential. This means that if the NAS server's VSS management port setting is changed, any existing VSS credentials for that server must be removed and new credentials must be created. If a DNS name is used for a NAS server, then changes to the server's IP address alone will not require removing and recreating the credential.
A VSS credential has limited rights on the server: it can only be used to perform VSS-related operations using the VSS management interface. In particular it cannot be used to gain access to the normal NAS server management console, either locally or remotely.