Thanks for writing in! Portfolio is a custom post type (similar to blog posts), so by default you cannot edit it using Cornerstone or Visual Composer plugin. When you add a portfolio item by filling it's details, those items will be displayed on a Portfolio layout.
You can see portfolio documentation on our knowledge base section.
However with recent changed to X theme, it is now possible to create custom portfolio items using Cornerstone editor. To do that, first you need to enable that feature by head over to X -> Settings -> Permissions -> General -> Portfolio area.
Hope that helps.