Both the WebM file and the VTT file can be read & muxed by mkvmerge 28.2.0 just fine here. The resulting file plays fine in VLC; subtitles are shown.
Did you right-click & "save as" perhaps? That way you might end up with HTML code being downloaded, not the actual files.
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Latest MKVToolNix is v83.0
If I ever ask you to upload something, please use my file server.
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