I know this feature still exists, why can't I find it after the redesign?
Watch the video at its own page (should include
http://www.youtube.com/watch?v=iwINBN0DHugin URL). If you are watching it on a channel, click "View comments, related videos, and more" beneath the video.
Click inside the comment textbox directly below the thumbs up/down buttons. (says "Respond to this video." inside it when you first visit.
Click the "attach a video" link to the right of the comment box. The page URL should include
Choose from "Record a video", "Choose a video", and "upload a video" in the grey bar and follow the instructions in the box below. When posted, your video response will appear in the "Video responses" section under the video.