The Caption Generator lets you create, synchronise and edit .vtt captions.
You can import a text (.txt) file and it will 'rough cut' your file into sensible, sensibly timed chunks. Then you can edit the captions to tidy them up, run them as a rough cut before synchronising them, and then, once you're happy with the words, you can synchronise them in real time with your video. Once you've done that, you can replay the timed file to make sure it's OK, make any further edits, and then download the finished file.
If you want, you can import a .vtt file to edit and synchronise it. And if you want to create your captions from scratch you can do that too.
So where would you like to start?