I understand that WebVTT has been adopted as the official subtitle format for WebM videos. I have a WebM file with such subtitles embedded in them, as reported by FFmpeg:
Duration: 00:01:35.98, start: 0.007000, bitrate: 458 kb/s
Stream #0:0: Video: vp9, yuv420p, 720x540, SAR 1:1 DAR 4:3, 23.98fps, 23.98 tbr, 1k tbn, 1k tbc
Stream #0:1(eng): Audio: opus, 48000 Hz, stereo, fltp (default)
Stream #0:2(eng): Subtitle: webvtt
In selecting the track from the Subtitle menu it appears that VLC 2.2.1 does not support embedded WebVTT, though. The code where it was added seems more for external subtitle files.