Based on the description from the article
Vendoring means taking it one step further: you commit all your dependencies’ source code into your own repo so the act of cloning the project gets you literally everything.
Forking is when you make a clone of a git repository, so you would check out a copy of the source code from your dependencies' repository, and then push them as their own repositories to Codeberg or wherever.
Vendoring is taking the result of your dependency installation, like your node_modules folder, and checking it in to your own repository. So, instead of 15 or whatever repositories with other folks' source code, you have just one repository with the built libraries checked in.
The benefit to forking is you can rebuild from source if you so choose, but the Vendoring means taking it one step further: you commit all your dependencies’ source code into your own repo so the act of cloning the project gets you literally everything. is it's a lot of repos to keep track of. The benefit to forking is you just reinstall the dependencies and check in the result, but the downside is your single repository is huge and takes forever to clone.