Example of enabling the VNC server on an iDRAC and setting the password:
racadm -r 10.1.1.200 -u root -p calvin set idrac.vncserver.enable Enabled
racadm -r 10.1.1.200 -u root -p calvin set idrac.vncserver.Password calvin
Note: Default port for VNC on the iDRAC is 5901
Connection example can be found here.