Hi Friend,
Below resources can give you more insight with respect to NFS on VDDK
This question may give you some more details on the same
Why is the backup throughput very low during the full backup?
If you are performing a full backup of virtual machines which have thin provisioned disks on NFS datastore, the backup throughput may become very low. VMware does not support the retrieving allocated blocks on NFS volume. Therefore, if a NFS datastore has a thin provisioned disk, the software reads the complete disk during the full backup. This reduces the backup throughput during the full backup. In case of incremental backups, software uses Change Block Tracking (CBT) and thus reads and backs up only the changed data.
For more information, refer to “Changed Block tracking on Virtual disks" section in the following document:
http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vddk51_programming.pdf
I am digging more into it. I will come back with some more details.