$ ffmpeg -hide_banner -v verbose -i 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlelist_leng.m3u8' -map 0:s -c:s copy -f webvtt -y out.vtt
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_979.webvtt', offset 0, playlist 0
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_980.webvtt', offset 0, playlist 0
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_981.webvtt', offset 0, playlist 0
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_982.webvtt', offset 0, playlist 0
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_983.webvtt', offset 0, playlist 0
[hls,applehttp @ 0xcf2da0] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_984.webvtt', offset 0, playlist 0
    Last message repeated 1 times
Input #0, hls,applehttp, from 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlelist_leng.m3u8':
  Duration: N/A, bitrate: N/A
  Program 0
    Metadata:
      variant_bitrate : 0
    Stream #0:0: Subtitle: webvtt
Output #0, webvtt, to 'out.vtt':
  Metadata:
    encoder         : Lavf57.31.100
    Stream #0:0: Subtitle: webvtt
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
Could not write header for output file #0 (incorrect codec parameters ?): Immediate exit requested
Exiting normally, received signal 2.