Why would you use RDMs instead of giving the LUN to the hypervisor and formatting with VMFS? After creating a datastore, use a SCSI controller on the guest and ....
As far as I can tell, the VSAN is a way to construct that kind if low-latency storage using local storage on hosts. Once vSphere sees the storage, though, it's just a matter of creating the datastore and using a SCSI controller in the guest again.