Maybe I should read the doc not just search on the forums.
I just saw this:
Backup fails if VM is enabled with VMware Fault Tolerance
If a VM has fault tolerance enabled, the backup will fail. This is expected behavior. VDP does not support backing up VMs that have Fault Tolerance enabled.
While that stinks to high heaven, I suppose it answers my question.
So is this something that's planned to be supported? It would be nice to use VDP for truly mission-critical VMs, which are the ones one would naturally use FT with...