Initialize videojs player with nuevo and thumbnails plugin. Provide vtt URL for thumbnails src option.
<script>
var player = videojs("player_1");
player.nuevo({ option1: "value1", option2: "value2" })
player.thumbnails({src:"../examples/assets/vtt/demo.vtt"});
</script>
To learn what are thumbnails images or thumbnails single sprite image, and to learn what VTT file structure should be, please visit VTT Thumbnails dedicated resource. If you login on Nuevodevel website you will also gain access to download bash sprite/vtt generator free script.