That's rather unlikely. Even if you had all the source code, not just the GPL parts, and were able to build and package the firmware in a way the TV would be ab

That's rather unlikely. Even if you had all the source code, not just the GPL parts, and were able to build and package the firmware in a way the TV would be able to accept, they're probably using some form of build signing to prevent that. But if you want to use Kodi, you likely already have some kind of media server, so something like Jellyfin/Emby/Plex wouldn't be too much more work and is definitely within the realm of possibility. SmartCast apps are just websites and Vizio gives you a companion JavaScript library for interacting with the TV and getting commands from the remote that could be bolted onto Jellyfin/whatever. Documentation is likely only available to big developers under an NDA, but the API is not exactly difficult to figure out (I can't give any further details due to an NDA completely urelated to Vizio :D)