Virtual Machines may have storage capacity mapped either as virtual disks or raw device mappings. Raw device mappings are Luns that do not build a datastore. Those Luns are directly passed to the virtual machine without any virtual layer inbetween. Raw Device mapped Luns cannot just be failovered like datastores as the virtual machine looses the connection to such devices if you would just swap the storage replication.

vfailover takes care of that extra work. It removes the current raw disk within virtual machine unregister process and adds the replication raw disk on the second site during virtual machine register process. vfailover not just keeps track of the raw device mappings but also scsi controller settings.