No failed or in failed or failing?
1. Check syllogism and /etc/vx/dmpevents.log for any h/w SCSI errors
2. Run dd do read a large trunk of data to test if the disks are ok
3. Replace the faulty disks, if diagnosed
4. If disks are ok, run
vxdisk set failing=off <disk name>
Run vxdisk rm disk_name for the mentioned disk twice. It should remove entries and then run vxdctl enable. It should give only one entry this time. If both entries appear again, you might need to go for device clean up or reboot ...