Visual Composer is a premium plugin which can be automatically updated via a purchase code (not the theme purchase code, but the plugin purchase code).
You don`t have to buy it, because you earned your right to use it for free, within our theme.
Same thing for RevolutionSlider, Ultimate Addons, Visual Composer and Essential Grid addon.
However, you`re not able to make automatic updates of the plugin, because it requires a licence key, which can be obtained only by purchasing it. You don`t have to, because plugin updates are managed by us, with each theme update, before ensuring that the new version is working fine with the theme.
That`s how things work.