While I appreciate the sentiment of the request and am a fellow fan of avoiding frameworks when possible, there are several reasons why I don't support the tag proposal.

• Practically speaking, JS is almost never used in pure vanilla form with absolutely zero external libraries. Using, say, axios, howler.js, lodash or a jQuery accordion here and there in an app is still more or less in the spirit of "vanilla" in my book. It's only non-vanilla when the libraries and frameworks begin to dictate the structure of the app (say, React) and/or nearly all DOM manipulation calls are done through wrappers (say, jQuery, React, etc).
• Even your hard-line definition leaves a lot of ambiguity: does Node.js count as "vanilla"? If so, I can't tag vanilla if I use Express or SQLite? What about TypeScript or a vanilla project that has to do with a bundler like Parcel or a test framework like Jest or Mocha?
• Practically, there are millions of JS questions, so even with a precise definition, tagging everything accurately would probably be an enormous undertaking.
• The definition is vague enough that new questions are sure to mess it up even if we allow the miracle of the old content getting tagged properly.
• Even if the tag usage is crystal clear and we grant that the library becomes tagged thoroughly enough to be useful, people don't really read or adhere to the usage and it'll be a mess anyway.
• It's unclear whether it'd be used in addition to or in place of the regular JS tag. Questions that have a lone React tag and no JS tag are sometimes a pain to find and deal with. Same for Rails without Ruby, Pandas without Python, Python-3.x without Python, etc. The prospect of another such tag is not exciting.

Conclusion: What constitutes "vanilla" isn't as universally obvious as clearer tags like, say, "React". Too much effort without enough of a clear-cut value proposition.

It may be a fool's errand, but here's a search that I quickly tossed together:

https://stackoverflow.com/questions/tagged/javascript+-reactjs+-jquery+-angular+-vue.js+-vuejs3+-solid-js+-next.js+-angularjs+-electron+-discord.js+-discord+-typescript+-socket.io+-nuxt.js+-nest.js+-react-native+-cypress+-three.js+-apollo

Whenever you see a tag that doesn't meet your definition of vanilla, add it to the query using the -[non-vanilla-library] filter syntax. Although you'll never get rid of all frameworks, you should be able to get most of the benefits of a full-fledged tag.

This is VanillaJS (unmodified):

// VanillaJS v1.0
// Released into the Public Domain
// Your code goes here:

As you can see, it's not really a framework or a library. It's just a running gag for framework-loving bosses or people who think you NEED to use a JS framework. It means you just use whatever your (for you own sake: non-legacy) browser gives you (using Vanilla JS when working with legacy browsers is a bad idea).