To Gain root access on VIO server:
Login as user padmin into VIO server and type the command,

$ oem_setup_env    
Now you will get the root credentials (without even been asked for a password).
Note: According to IBM its not recommended to work as root on VIO servers.
But, By default the ioscli commands are not available for the root user. All ioscli commands are in fact calls of /usr/ios/cli/ioscli with the command as argument. You can check this if you list the aliases of the padmin user in the VIO server
So you can use all ioscli commands as user root by appending /usr/ios/cli/ioscli in front of the command you type. 
For Example,
Instead of lsmap -all you would type,
root@viosrv1# /usr/ios/cli/ioscli lsmap -all
you can set an alias to make your job easier,
alias vios=/usr/ios/cli/ioscli
Now, you can even type,
 root@viosrv1# vios lsmap -all