You have not said now your storage is configured on Vmware Block or Nas access?
NFS has some tools you could use but they may not be helpful.
Have you looked at the problem from the ESX side? Have a look at this
http://www.vmware.com/pdf/esx2_using_esxtop.pdf
VMware ESX Server 2
Using esxtop to Troubleshoot Performance Problems
If you are still struggling please describe your filers, connectivity and ESX disc configuration.
Welcome to the forum.
Brendon