Hi John. Great question.
Exchange 2010 and 2013 reads and writes at 32k 'pages' on a storage system, therefore you want to use a Nimble Performance Policy which will assign you 32k blocks. (source: Exchange Volume Block Size). This confusion is something we see often (normally with SQL server, where people believe it's a 64k block, but in fact it's 8k pages as part of a 64k extent).
The Exchange BPG references NWT/NCM for Windows if you wish to use Exchange integrated & consistent snapshots. You're able to use VMDK within VMware (with NCM for VMware) but just be aware that it will require VMware based consistency snapshots which aren't the best in the world.
According to this TechNet article (Exchange 2013 storage configuration options: Exchange 2013 Help) a block size of 256KB or greater is recommended for Exchange 2013 databases. However, my CS300 doesn't have a performance policy for Exchange 2013 and the Exchange 2010 performance policy has a block size of 32KB pre-configured. Everything I've been able to find on Nimble's site indicates that 32KB is also optimal for Exchange 2013, but since this flies in the face of Microsoft's recommendation I was hoping to get a little clarity on the issue. Is the recommendation different because of CASL? Any information is greatly appreciated. Thank you.
Edit: Also, the Exchange Best Practice guide strictly addresses using NWT for Exchange iSCSI connections. Is it recommended to use VMDK's with NCM for Exchange database volumes?