In vSphere 5.1 you have the ability to create a specific VMKernel for vSphere Replication (just like you do for vMotion or FT by checking the check box), however the traffic still runs over the management NIC. To create that VMKernel and have the vSphere Replication check box you'll need to log into the vSphere Web Client and create the VMkernel. However with that said even with that new VMKernel created my VR traffic is still currently running over my Management NIC.
I have found that in 5.0 you can create a static route on the Protected Site's Hosts: http://communities.vmware.com/message/2128647#2128647 to keep it off of the Management NIC. I'm currently working with VMware support on this and some other vSphere Replication issues so I'll reply back if they give me anything other than the static routes.