Because there is no documentation on how to support VSS on the file system level.
There is documentation on how to access VSS, and plenty of it. But that's not the issue. The problem is how the file system is supposed to handle the VSS calls. There is NO documentation on this, in the wild. Any documentation that may exist is internal Microsoft documentation.
If by Samba, you mean Samba/SMB/CIFS/Windows Shares, then you're just connecting to the API. You're relying on the underlying drive that the SMB share resides on supporting VSS. This is the top level VSS stuff, we need the bottom/low level info, how you'd implement it on a different file system.
So, right now, we'd have to reverse engineer exactly how VSS interacts with NTFS, at the file system level. That is not a simple thing, at all. And it would be incredibly time consuming.
If you mean a specific software, could you link it?
Back up the underlying disks in the pool, not the pool drive.
As for restoring .... basically the same.
That or used something file based, or a sync utility (such as AllWays sync, good sync, free file sync, synctoy, etc).