Actually, the digest file is used by the CBRC cache mecanism so if you restore the "data" vmdk without the digest vmdk, you need to recreate it. Even if the digest is a bit dirty, it could still bring cache hit so it would be nice to back it up. BTW, the digest file path is present in the "data" vmdk file but the VM is able to boot without it, you just have no cache.
I working on a big VDI infrascture to push this feature and size the VM are dedicated per user and backed up with veeam, this could be a NO GO for the customer because each time a VM would be restore, the digest file recreation would be needed.
Thanks for your help