Network File Copy (NFC) provides a file-type-aware FTP service for vSphere components. ESXi uses NFC for operations such as copying and moving data between datastores. Since it uses FTP for transfers it is considerably faster.
I researched further into usage of the NFCFileManager object and found that the nfcService is used in the transfer which is an internal API which is not exposed through the sdk. Hence there is no way to access the object. Also internalvim25 is the handle to APIs internal and external. VirtualDiskManager is an object which internally calls the NFCFileManager but only provides partial functionality. Although file transfers to the datastore through VIClient are managed by the nfcService it cannot be accessed through the API. To add further virtualDiskManager doesn’t provide functionality to transfer files to the datastore from local system but files can be transferred from one datastore to another which uses the NFCFileManager internally to complete the transfer.