Most video editors and upload forms want SubRip. WebVTT is the web format, and plenty of software still will not read it.
Open a project in the editor and go to Captions. You can start from a video you are captioning, or from nothing at all if you only need the file converted.
Bring in the VTT file. The cues and their timings are read in as they are, so nothing is re-timed and nothing is re-transcribed.
Export as SRT. You can also burn the captions into the video instead if you would rather they could never go missing.
WebVTT exists because browsers needed a caption format with positioning and styling. That makes it the right answer inside a web page and the wrong answer nearly everywhere else: a lot of desktop editors, broadcast tools and platform upload forms read SubRip and nothing else.
Going this direction the conversion is a simplification. The cue text and timings carry over exactly. What does not carry over is anything WebVTT-specific: cue positioning, alignment settings, and any styling attached to a cue are not expressible in SubRip and are dropped.
For the overwhelming majority of caption files, which are plain text at the bottom of the frame, nothing is lost at all.
Cue settings such as line, position and align have no SubRip equivalent, so a caption you had deliberately placed at the top of the frame will come back sitting at the bottom with the rest.
Voice and class tags, the ones that mark who is speaking or attach a style, are removed as well. If the speaker labels matter, put them in the text itself before converting rather than relying on the markup.
Only WebVTT-specific features: cue positioning, alignment and any styling or voice tags. The text and the timings are unchanged, and a plain caption file loses nothing.
WebVTT was designed for browsers, and a lot of desktop and broadcast software never adopted it. SubRip is the older, simpler format that almost everything reads.
Free, in the browser, and the captions can be burned into the video in the same place.