Jump to next/previous subtitle is not implemented. You can find/open a feature request ticket at
https://trac.videolan.org/vlc or you can hire someone to implement it in VLC.
You can help yourself and write VLC Extension Lua script with dialog box with buttons to jump to previous/next subtitle. There are already several extensions working with srt subtitles:
http://addons.videolan.org/index.php?xcontentmode=903
https://forum.videolan.org/viewtopic.ph ... 20#p377455
Some subtitle editor would really be useful here to jump to previous/next subtitle, to copy the current text, ...