Flux is an architect pattern to build React application. So you can use Backbone models and collections inside your stores to fetch and store data.
And if want to use just React’s Virtual DOM feature, there is no need to use react.js. There are a lot of libraries, adding Virtual DOM feature to your application (https://github.com/Matt-Esch/virtual-dom).
My recommendation: if you will use Flux pattern I strongly recommend you to use http://facebook.github.io/immutable-js/ (may be coupled with http://ampersandjs.com/; don’t forget to define your custom sync function if you are building isomorphic application). Basically there are no any advantages using backbone models with React (backbone is heavy, it needs underscore, which is slow; I use https://lodash.com/ instead ).