Never mind that.
after googling for release notes and finally found it on 5.1.21 release
- The disk virtual machine restores a think disk in the vCenter. This issue is resolved in this release. When restored as a new VM, the VM now restores as a thin disk in the vCenter.
Reference : http://www.vmware.com/support/vdr/doc/vdp_5121_releasenotes.html
I'll upgrade my VDP to latest version and test again.
