Custom VRs are only allowed in certain Extreme Switches (you would have to check the manuals, but I think it is the high end only (x460, x670, BDs etc...) If you are using netsight to download images I would recommend using the scripts that are provided. In Inventory manager you select the MIB override method (right click) and select script. Then you select the script that you need for your switch, (SCP or TFTP , VR-Default or VR-Mgmt)