You can use replication seeds if a duplicate file is found for the virtual machine on the destination datastore. vSphere Replication compares differences and replicates only the changed blocks.
To avoid network bandwidth consumption for the amount of data that has to be replicated on initial full synchronization, vSphere Replication allows you to copy your virtual disk files to the remote datacenter and point those as replication seeds during configuring replication. vSphere Replication compares the differences and replicates only the changed blocks.
When configuring replication for a virtual machine, vSphere Replication looks for a disk with the same filename in the target datastore. If a file with the same name exists, vSphere Replication prompts you with a warning and offers you the option to use the target disk as a seed for replication. If you accept the option, vSphere Replication compares the differences and replicates only the changed blocks after the virtual machine replication is fully configured and enabled. If you do not accept the prompt, then you must change the target location for your replication.
The source virtual machine must be powered off before downloading the vmdk files that are used as seeds for the replication.