I recommend RDMs over NPIV. Please have a look at the following blog posts and see if this helps.
FitProgrammer@Work: Using NPIV port for RDM disk access in VMware vSphere
http://blogs.vmware.com/vsphere/2011/11/npiv-n-port-id-virtualization.html
http://vmwarehints.blogspot.in/2012/11/enable-npiv-in-vsphere-enviroment.html
http://wojcieh.net/how-to-configure-n-port-id-virtualization-on-vmware-esxi/