To reduce the network traffic that is generated by data transfer during the initial full synchronization, vSphere Replication allows you to copy virtual disk files or use files that already exist in the target datastore, and point those as replication seeds when you configure a replication.
vSphere Replication compares the differences on the source and target site, and replicates only the changed blocks.
When, during replication configuration, you select a target datastore for the 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 existing disk file as a seed for the replication. If you accept the option, after the virtual machine replication is fully configured and enabled, vSphere Replication compares the differences and replicates only the changed blocks. If you do not accept the prompt, you must change the target location for your replication.
If you plan to copy files from the source to the target datastore, the source virtual machine must be powered off before downloading the vmdk files that will be used as seeds for the replication.