NFS is not a "VMware thing" so I would think the better place to find this answer would be the Internet for default, but more importantly, your specific configuration files. Perhaps the config was modified to use non-standard ports. It also depends on which version of NFS you are using. With that said, you should be able to mount an NFS datastore from behind a firewall given the policy is properly implemented. I would also consider checking the firewall to make sure that it can handle the traffic flow and won't cause a bottle neck for your VMs.
