What is a difference between SAN and vSAN?

SAN is a storage area network . While (VSAN) is a virtual storage area network which is a logical partition in a storage area network . VSANs allow traffic to be isolated within specific portions of a storage area network.

VSAN is a  logical partition in SAN, used for limiting or isolating the data traffic within a specified part of SAN. 

VSAN and distributed disk (I believe) will be the way of the future.  Just look at the way google/FB/amazon build out their datacenters.

There are some alternatives to native VSAN if you want a more mature product.  Look at Nutanix or Simplivity for a similar architecture to VSAN and a building block approach to VMWare.

If I was building a new VMWare infrastructure from the ground up. I'd probably go the VSAN/Nutanix/Simplivity route.

One "advantage" of the VNX is that you can share your Windows shares directly from that rather than from a VM.  You would want to do this as it uses fewer resources and is more reliable.