Hi Marvin,
The firmware required for vRIot, is not available on support site 3.6 and 5.1.1 are for vsz and sz100 respectively and I have been told if you have IoT module on the APs then you need to use these firmwares on you wireless controller, but it depends.
If you have vRIoT 1.3 image pls share it, it wouldn't be large one.
Regards,
Abi