With vSphere Replication 5.1 you can replicate within one instance of vCenter between hosts. No second vCenter is required. However, you should keep in mind that in order to recover machines you need the vCenter to be up and running. Furthermore, vSphere Replication appliance is allowed one per vCenter server.
To your second question, see: http://www.yellow-bricks.com/2012/09/21/can-i-protect-my-vcenter-server-with-vsphere-replication/