Do you mean that your VMWare datastores are NFS, and the guests are writing to their "local" VMDK filesystem? Or that your VM guests are mounting NFS from NetApp?
You can get per-client NFS usage from NetApp, but if your datastores are NFS, all you'll see is ESX host access, not individual VM access. That being said, the vSphere client gives you pretty good per-guest IO information.
To get per-host NFS stats, you need to enable the nfs.per_client_stats.enable option, then check out nfsstat -h.