Themes purchased from ThemeForest cannot use the normal WordPress notifications and updates system. This is not something we can control, we do not operate the ThemeForest website.
On your ThemeForest downloads page you’ll see an option you can enable that will send you an email every time a theme is updated.
Alternatively, Envato has made a plugin you can use to help keep themes purchased from ThemeForest up to date.
You can download it from here. → https://envato.com/market-plugin/
(Envato is the company that owns and operates the ThemeForest website.)
If you need them, instructions for updating the theme are here → http://para.llel.us/support/groups/vellum/forum/topic/latest-version-vellum/
Thanks.