Thank you GilShrewmouse
For clarity: there is zero need to touch the HTML file. All you guys need to do is download the package with the two files, unzip, replace the content of the vtt file with yours, rezip and upload. Hope it helps!
Also, remember that SUBTITLES CAN ONLY BE SEEN ONLINE. If you open the html on your local system, the browser will simply refuse to load them (safety measures)