Data Fabric Manager is an obsolete name for something that is now a function of OnCommand Unified Manager (also known as OnCommand Core). Internally in OnCommand Core you will see references to "dfm" so a lot of people still use that name unofficially.
OnCmd is not required to perform SnapVault or SnapMirror. It provides an external server which can be used to manage and monitor SV/SM (and local Snapshots) across multiple storage systems from a central console. Rather than using the snap* schedulers built into ONTAP you can use OnCmd's scheduler to initiate snapshots and replication. From the point of view of the storage controller there is no snap* schedule and every snapshot, snapmirror update or snap vault update is a "once off manual operation". Using OnCmd policies is meant to allow you to enable and modify the data protection configuration on multiple volumes on multiple systems more easily. It also allows you to do things like send email or SNMP alerts if snap* protection is out of date, and automatically provision secondary volumes or qtrees using policies. This functionality requires a "standard" (not "express") installation of OnCommand Unified Manager and an ONCMD_CORE license key.