ffmpeg -i http://tvh_url -map 0 -c:s webvtt -acodec copy -vcodec h264 -b:v 2M -pix_fmt nv21 -f hls -hls_time 10 -hls_list_size size 10 -hls_flags delete_segments -hls_segment_filename 'file%03d.ts' out.m3u8

Output #0, hls, to '10':
    Stream #0:0: Video: h264 (h264_v4l2m2m), nv21, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 2000 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc57.24.102 h264_v4l2m2m
    Stream #0:1(dut): Audio: mp2 ([4][0][0][0] / 0x0004), 48000 Hz, stereo, 256 kb/s
    Stream #0:2(dut,dut): Subtitle: webvtt, 128 kb/s
    Metadata:
      encoder         : Lavc57.24.102 webvtt
Output #1, hls, to 'out.m3u8':
    Stream #1:0: Video: h264, none, q=2-31, 128 kb/s, SAR 1:1 DAR 0:0, 25 fps
    Metadata:
      encoder         : Lavc57.24.102 h264_v4l2m2m
    Stream #1:1(dut): Audio: aac, 0 channels, 128 kb/s
    Metadata:
      encoder         : Lavc57.24.102 aac
    Stream #1:2(dut,dut): Subtitle: webvtt, 128 kb/s
    Metadata:
      encoder         : Lavc57.24.102 webvtt
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (h264_v4l2m2m))
  Stream #0:1 -> #0:1 (copy)
  Stream #0:2 -> #0:2 (dvb_teletext (libzvbi_teletextdec) -> webvtt (native))
  Stream #0:0 -> #1:0 (h264 (native) -> h264 (h264_v4l2m2m))
  Stream #0:1 -> #1:1 (mp2 (native) -> aac (native))
  Stream #0:2 -> #1:2 (dvb_teletext (libzvbi_teletextdec) -> webvtt (native))
Error while opening encoder for output stream #0:2 - maybe incorrect parameters such as bit_rate, rate, width or height