I don't really hear much about Vox, and nobody I know really uses it save for 6A folks, former folks, and a few people who got dragged in early. I decided to look today to see if I could find any interesting usage data. As far as I can tell, they got fewer than 5000 public posts in the last 24 hours. In contrast, 200K LJ users have been active in the last 24 hours. That's definitely an apples-and-oranges comparison for a lot of reasons, but even if you double the Vox number and halve the LJ number, that's still a factor of 10 in LJ's favor. A factor of 20 is probably closer to real.
It looks like Vox isn't exactly catching on.
 The Vox number counts public posts, while the LJ number is active accounts. The Vox number will be high because some people post more than once, and low because it isn't counting private posts and other activity. It was measured by going backward through all public posts to find out how many pages it took to get to approximately this time yesterday (around 140, or 2800 posts), and relies on that page being sorted correctly, which is questionable at best.
The LJ number counts active accounts. It will be high because it counts all activity, and low because some people will post more than once.