If you not use the third party library, you will make you own library, plugin or framework which functions just like third party.

plugin, library and framework solve or hidden the issues of Vanilla Javascript

Even simpliest webapp functionality like client-side todo list require enormous boilerplate when using only standard library. Standard library just don't have abstractions that are useful in almost every web app.

With vanilla JavaScript you need to take in account the different browser implementations and be unable to target older browsers without newer featurea unless you write a polyfill library, and then it stops being vanilla JavaScript.