We'll launch this Nico Nico Douga integration soon on Vox, hopefully early next week. What's nice about this integration is not only that you can embed the video, but also that the video is actually added to your vox Video library along with YouTube.
Because Nico Nico Douga doesn't provide a search interface for logged-out users, the only way you can do this would be to use our VoxThis bookmarklet. Install it now to your browser so you can start blogging Nico Nico video as soon as we launch the feature. Stay tuned!