When mapping shortcodes using vc_map, you can assign icons to your new Visual Composer element.
If you created a container element, that wraps around other elements, the child element icon will currently be overwritten with the parent icon. A fix is apparently on its way :)
Currently the only way is to skip the icon option completely and use pure CSS for that.
You can enqueue a CSSs file for the admin through a vc_map option “admin_enqueue_css“.
The CSS targets the icon of the displayed element in the editor and the icon when adding new elements to the layout.