Let's say I wanted to deploy a new vSAN cluster to replace my older traditional infrastructure. would I be able to relocate my older equipment (3 servers, and a SAN) to another Site and implement storage replication between a vSAN datastore and a VMFS datastore? \
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Yes of course - vSAN doesn't particularly care whether the source/destination of the vSphere replication is VMFS datastore or another vSAN cluster and vice-versa.