What about other IO on the aggregate?
Any other volume activity (snaps, resize)

I feel your pain.  I've had one single VM cause IO spikes and thus latency.  I had this happen when I moved the VM from an NFS datastore to a VMFS datastore to get more IO potential. Well, that extra IO was too much of a piece of the pie.

The other night I had all NFS/CIFS IO completely stop for a couple of minutes during a volume resize.

I love NFS+VMWare+NetApp but sometimes I hate it as well.

Brian