I just discovered this last night and I was doing some checking around. I haven't tried it yet, but I do think it might be possible to install the app on the Vox and buy books from the Google Play book store. I haven't actually purchased any books yet from the Google book store, but I do have some free ones in my Google "library". The Vox is not an officially supported device for their app, but one could download the epub and side load it to the Vox, from what I can see, for use with other reading apps.
Just another shopping option. I've been doing price comparisons between various e-book vendors, and the Google book store looks competitive with Kobo on some titles. Other vendors (Sony and Kindle) are charging more for the titles I've been looking at, but I Kindle tends to carry titles that Kobo doesn't have available.
Has anyone tried the Google Play book app on the Vox?