Have you seen https://www.youtube.com/watch?v=FfUqIwT8KzI ? It's a good intro to understand how it works.
Can you switch to NFS in your FreeNAS? You'll reduce a LOT your required space to make backup/snapshots
All delta mechanism will require to keep one snapshot. On a thick pro storage, you'll need to roughly double space needed for your VMs. On a thin pro storage, it's merely few extra %.
Only the first snap will require doubling space, not extra snaps.
VSS: it's removed by Citrix in 8.1, so it will be also removed in XCP-ng 8.1. However, we are working on another way to make consistent backups on Windows. Until then, you can use "Offline snapshot" option in Advanced view of your backup job.
Linux VM: they are aware of a snapshot because of the disk driver they have by default. No problem at all without VSS, it's somehow "bundled".
