The vers parameter does not have to be specified unless you would like to override the default. The z/OS NFS client will communicate at the highest protocol level supported by the server, by default. Verify the protocol level for tuning of read or write buffer size (wsize, rsize). Use nfsstat -m /mountpoint command and look for vers(n) in the report or use rpcinfo -p servername command and look for "100003 4 TCP 2049 nfs" in the report.