What is VanillaJS?

Using "VanillaJS" means using plain JavaScript without any additional libraries like jQuery.

People use it as a joke to remind other developers that many things can be done nowadays without the need for additional JavaScript libraries.

Here's a funny site that jokingly talks about this: http://vanilla-js.com/

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).