The subtitles menu should always be available for adding titles ... or deleting those that are not embedded into video. Once embedded, on line 21 of video, they cannot be separated out. However, the player may be able to suppress them. If they are open captions - subtitles - they are superimposed as an integral element of the video.
Don't believe that VP can display captions that are embedded in line 21, but don't quote me.
When creating a project, save the subtitles as an SSA file. then export the project sans subtitles. Then the project can be restored, and the subs added via import of the SSA file.