$ ffmpeg -hide_banner -v verbose -i 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlelist_leng.m3u8' -y out.vtt
[hls,applehttp @ 0x1030d20] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_819.webvtt', offset 0, playlist 0
[hls,applehttp @ 0x1030d20] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_820.webvtt', offset 0, playlist 0
[hls,applehttp @ 0x1030d20] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_821.webvtt', offset 0, playlist 0
[hls,applehttp @ 0x1030d20] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_822.webvtt', offset 0, playlist 0
[hls,applehttp @ 0x1030d20] HLS request for url 'http://vlive.hls.edgesuite.net/kr026/1200.stream/subtitlechunk_leng_823.webvtt', offset 0, playlist 0
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
    Metadata:
      encoder         : Lavc57.34.100 webvtt
Stream mapping:
  Stream #0:0 -> #0:0 (webvtt (native) -> webvtt (native))
Could not write header for output file #0 (incorrect codec parameters ?): Immediate exit requested
Exiting normally, received signal 2.