I looked at the page and I think the documentation is a little lacking.
I only understand what VanillaJS is supposed to do by implication, by looking at the PlainJS sight and then extrapolating that this must be competition.
I found the documentation link on VanillaJS on the bottom of the page. It seems VanillaJS is a project of Mozilla, or adopted by them. Very nice for a browser maker to spend time documenting a library so thoroughly. I would say the docs are almost better than jQuery's docs.
On the other hand, I like jQuery more for the support community :)
I only understand what VanillaJS is supposed to do by implication, by looking at the PlainJS sight and then extrapolating that this must be competition.