Hi,
I dont understand why you need to monitor data change rate for protected virtual machine using vSPhere Replicate. If you think of performance hit because of replication , then I suggest you to go through this blog which explain how replication work.
http://blogs.vmware.com/vsphere/2011/10/how-does-vsphere-replication-work.html
Also here is link for Advanced vSphere Replication Options for Single VM Replication Performance
http://blogs.vmware.com/vsphere/2012/06/increasing-vr-bandwidth.html
Regards
Mohammed