You're providing a library. That library has dependencies (although it shouldn't). You've written that library to work against a specific version of those dependencies. Vendoring these dependencies means shipping them with your library, and not relying on your user, or even worse, their package manager to provide said dependencies.
I don't know what industry you work in, who the regulatory body that certifies your code is, or what their procedures are, but if they're not certifying the "random library repos" that are part of your code, I pray I never have to interact with your code.
You're providing a library. That library has dependencies (although it shouldn't). You've written that library to work against a specific version of those dependencies. Vendoring these dependencies means shipping them with your library, and not relying on your user, or even worse, their package manager to provide said dependencies.
I don't know what industry you work in, who the regulatory body that certifies your code is, or what their procedures are, but if they're not certifying the "random library repos" that are part of your code, I pray I never have to interact with your code.