If you wanted to try the esxi route
I found this info on unRaid NFS
http://lime-technology.com/wiki/index.php/FAQ#How_do_I_configure_NFS_mounts.3F
Then in vcenter for esxi add a datastore with the network file system option
give esxi the unraid servers nfs address
tell it where to mount it on esxi
and give the datastore a name, like images
Then on your fog vm, shutdown, edit settings, and then add a new virtual disk from that data store.
Then mount that as /images in your /etc/fstab. You might have to format it before you can mount it with mkfs ext4 or something of that sort.
If that overview isn’t helpful and you want to try this method, I’ll test it out a little and document the process more thoroughly when I get a chance.