It's poking fun at how there's a new flavour of JavaScript framework seemingly every week: Angular, React, Vue, Mithril, etc. Each one has a killer feature or some new spin on how to write your JavaScript code.
The word "vanilla" is used here as an adjective to describe something that is "lacking distinction: plain, ordinary, or conventional" [0].
As such, the joke is that Vanilla JS is being presented as some amazing new framework, but it isn't anything more than regular JS that you would write. In a sense, it's the anti-JS framework, requiring no additional scripts or dependencies.
