what is VAAI ?? and how its work when using VNX File as storage ??

VAAI is VMware's vStorage API for Array Integration... It offloads some storage intensive operations from the hypervisor onto the storage array. Things like full copy hardware assisted move, zero space reclamation, thin provisioning, etc.

I don't do much with VNX File and NFS datastores,  but their is a VAAI for NFS Host Extension you can get on support.emc.com.

How about a look at the white paper for VNX file and VAAI ?

If you input “VNX VAAI” on the search box on emc.com the very first two search results are for VAAI with block and with file