Which PowerCLI version are you using?
The reason for asking, since 6.5.1, and if you are using PowerShell 3 or above, you use the module auto-loading feature.
Also note that the New-VICredentialStoreItem stores the encoded credentials indeed in a XML file.
But be aware that only the same user, and on the same station, where the entries were created, can retrieve and decrypt the credentials.
The VICredentialStore uses a Windows API for encrypting/decrypting, and that uses the current user and station to do the encryption/decryption.
Also the reason why the VICredentialStoreItem cmdlet do not work on PowerShell Core on a non-Windows platform.