Like they say in their FAQ it’s source available but not open source
No, GameVault is source-available, meaning the code is open for you to explore and modify for personal use. However, you may not use it for commercial purposes.
They also have a paid premium version which is required to use third-party clients, so there will be no heroic integration unless you pay
Never used Nixpkgs, but isn’t it a bit more advanced and not really for beginners?