This is just for me, I like that the video page is not cluttered with comments. I like the forum format with the ability to create thread for specific question or find a specific topic that may already have an answer to my questions. It would be nice though if the video can be linked to the forum discussion like if the title is regarding Lesson 55, the video would be there under the topic name or the first post.
What I see as downside to comment section under the video is that, with the video being a fixed content, old and new comments from current and new members will exist together and pile up like in Youtube that a question might get burried in the comments by new discussion before it gets an answer, if it'll even get an answer. And it might also eventually clutter up the page and slow it down... unless maybe if the comment section is set to purge the old comments at a set number of months, or when it reaches, say, 500 or 1000 comments but I don't know if vimeo have that capability.
Of course, that's, if the comment section becomes active like youtube. Right now though, if it stays at the pace of this forum, it's manageable.