I don't know about Vox. I watched the CEO talk at a Re\Code event of all places. He was very dodgy when talking about financials. They are not afraid to spend and have strong backing but I'm not so sure they can make back the mountains of cash they spend.
In any case, it's very interesting to watch how this unfolds.
The other issue is that millennials, especially technical and gaming are among the highest users of adblock which makes a material impact on ad revenue from the site. Either way, even doubling to 3M uniques per month is still a low number of overall users to sustain a single site.
It might work if part of a larger network, which is what will happen at Vox, but they seem to be hitting the saturation point too. Remember the Verge superbowl ad? It's nice to keep buying traffic and hiring writers as long as the VC money is there but it doesnt seem like they have sustainability figured out yet.