How do I add VTT subtitles to a MP4 video to view on an iPhone iOS?
Wow I finally found a solution. I found 
https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/subtitle_options
and
https://trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo
so I tried a few things and found this worked.


  C:\Users\Chloe\Videos>"\Program Files\WinFF\ffmpeg.exe" -i russian_ark.mp4 -i russian_ark.vtt -vcodec copy -acodec copy -scodec mov_text russian_ark_sub.mp4


Unfortunately WinFF doesn't have options for subtitles in the GUI. I don't know what mov_text is but luckily FFMPEG can understand VTT and convert to it and Apple can understand mov_text. I tried to use -scodec srt but it kept giving me an error track 2: could not find tag, codec not currently supported in container. Could not write header for output file #0 (incorrect codec parameters ?): Operation not permitted. I didn't want to hard-code the subtitles as described in the second link, but I was almost ready to.

Thanks to Insomniac Software for letting me know Apple can't do this automatically, not even with iTunes.