Now there's a step in the right direction! If I create a shortcut to VLC to launch it with that command-line parameter, then explicitly open the file from within VLC, it does indeed launch with the subtitle enabled
However, how can I just configure it to *always* open with the subtitles enabled (i.e. when double-clicking files directly)?
I tried changing HKCR/Applications/vlc.exe/shell/Open/command/(Default) from "C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" --started-from-file "%1" to "C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" --started-from-file "%1" --sub-track 0 (as well as reverse order for the parameters, "C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" --sub-track 0 --started-from-file "%1"). Neither case worked.