Unfortunately this seems to be service as a software substitute it would be really nice to have this as a free software program that can be run locally.
Interestingly if you run this detector on itself (as it invites you to do) you get a funny easter egg response that includes the statement "VibeDetect is an open-source tool built with Next.js and TypeScript. The code is available on GitHub — check it yourself to see how it works" along with a link labelled "View on GitHub (if it exists)" that goes to "github.com" with no repository. So I'm not sure what to make of that.
For my part I was unable to find the source code for this on GitHub searching with DuckDuckGo or Kagi. The site itself offers no information on who built it, either.