Here is little more info after i did some tests this morning
First create a role, I named it "test", with access to VS 6 and then select commands that you want to allow to this role. Note that ext_xxxx commands do require read-write option as they are expert type commands, so be careful with those. They will still be executed from clish shell not bash though.
add rba role test virtual-system-access 6
add rba role test domain-type System readonly-features interface
add rba role test domain-type System readonly-features route
add rba role test domain-type System readwrite-features ext_cpview
add rba role test domain-type System readwrite-features ext_top
add rba role test domain-type System readwrite-features ext_ping
add rba role test domain-type System readwrite-features ext_cphaprob
add rba role test domain-type System readwrite-features ext_netstat
add rba role test domain-type System readwrite-features ext_traceroute
Then add a new user, I called it "testing" and couple it with the newly created role
add user testing uid 0 homedir /home/testing
set user testing password
add rba user testing roles test
Now you will have a user that has access to VS6 clish with named commands
vsxext:0> show rba role test
Role
test
domain-type System
virtual-system access: 6
read-write-feature ext_cphaprob
read-write-feature ext_cpview
read-write-feature ext_netstat
read-write-feature ext_ping
read-write-feature ext_top
read-write-feature ext_traceroute
read-only-feature interface
read-only-feature route