Yes a greyed out vDisks size field is always an strong indicator that the vDisk is running on an snapshot. You have to look to he vmdk filename to be 100% sure because when using SATA controller for the VM its also greyed out when the VM is on. So check the filename.
For deleting a Snapshot you need free space! How much depends on ESXi version and of course the size of the snapshot. Please run ls -lisa and df -h from the command line.
Regards,
Joerg
