To elaborate on the more pragmatic answers: vendoring generally refers to adding the source code of a dependency to your source control repository rather than having an install step (or expecting libraries to be provided by system level package managers) as part of your development process.
In JavaScript these days, dependencies are usually loaded from npm so avoiding the bundling step for client-side code would naively mean replacing those dependencies with links to a third-party service like unpkg, which serves code published to npm for the browser.
As others have explained, vendoring in this case would mean shipping those dependencies as part of your application code so they are served from your own domain. Of course at the moment a lot of dependencies will likely simply not work if imported directly from the browser.
To clarify, it isn't vendoring to have an external script in your build output. Just because it's technical jargon doesn't mean there can't be a clear meaning of it.
I assume it means the same thing as to vendor anything else: to bundle dependencies as a part of your application instead of giving users "links" to the dependencies served by a third party.
