Is React a good introduction to the land of frontend programming? Or do I first need to familiarize myself with vanilla JS and go bottom up to React?
[1] http://bonsaiden.github.com/JavaScript-Garden/
With the JS quirks out of the way, I would recommend building something in React and muddle through things with SO/google.
The hardest part of frontend dev for an experienced backend dev is the CSS. CSS is conceptually simple but there's a LOT of random domain knowledge you need to get layouts you want. You can bypass a lot of it by using a sass framework (e.g. foundation or bourbon) at the cost of adding a build step and another abstraction layer on top of CSS.
This is a pretty great book
Alternate your time spent learning.
Sometimes study high-level, sometimes low-level. (Low-level, tongue-in-cheek, as in "JS is the bytecode of web applications.")
I think you can build a lot on top of React with minimal knowledge of js (so, yes, start), but you definitely want to go deep and understand how it works inside (so, yes, also study js).
As you're using a high level framework, you're interacting with that more than the details of JavaScript. So in essence you're learning 'React' not 'JavaScript'.
So just start building in React and the JS will come along naturally.