You might find the old
IDPF epub3 test file helpful.
They use the following code for a video with vtt subtitles:
Code:
<video controls="" width="278" height="240">
<source src="../video/screencast.mp4" type="video/mp4"/>
<source src="../video/screencast.webm" type="video/webm"/>
<track kind="subtitles" src="../video/tracks/screencast.vtt"
srclang="en" label="English" default="default"/>
If this text is rendered, the video element is not supported and the test fails.
</video>