Ran into a similar issue where add a vFlash Resource Resource completed successfully, but would not show in the Device Backing field. Trying to power on a VM configured with vFRC would fail stating the vFRC was not available. Trying to delete the partition using partedUtil delete /vmfs/devices/t10.ATAxxx would fail due to device being read-only. No other resources were using the partition such as scratch partition. I ended up attaching the SSD to a Windows 7 machine and used diskpart to zero out the SSD.
From Windows cmd:
1. diskpart
2. list disk
2. disk select 3 (this will vary, be SURE to select correct disk!)
3. clean all (zero's out entire disk, not specifying 'all' will only zero out the first and last mb. Doing just a clean did not work for me the first time.)
After attaching SSD back to host, run ls /vmfs/devices/disk to validate no partition exists for the SSD vFRC disk. Add SSD to vFRC completed successfully and showed as a Device Backing. VM configured with vFRC powered on.