You need to save the custom bookmarks:
Media > Save Playlist to File...
You can also edit bookmarks in the bookmarks manager to change their description (select bookmark > left-click > wait a second...) or you can edit saved playlist file in a text editor.
Media Library is a special playlist (ml.xspf), that is saved/loaded automatically in VLC: View > Playlist > Media Library
You can try VLC Extension:
Moments' Tracker