cbt first needs to be enabled on VMware side (be aware of VMware KB: Enabling Change Block Tracking in vSphere 5.5 fails). Unlike Veeam, NW won't enable CBT on hypervisor as that is something ESX admin should do and there might be reasons why they even do not wish to use it.  So, to my knowledge, you need to enable it yourself.  I would suggest to read http://www.virtualizationsoftware.com/using-vsphere-changed-block-tracking-emc-networker-backup/ too.