Everyone will probably yell EXTENSION!, but is there a way to create a new discussion programatically (easily)?
Someone mentioned that they use Vanilla as a Blog comment system. This is simply brilliant. I'd love to be able to include a php file and call a function that takes a few paramaters (title, date?, blog body? (summary?)) and spits back the URL of the discussion that was created with the same title (or Title [comments] or something to tag it as "related to some other thing on the site"). That way, this could be included at the bottom of a blog entry.
Really smart functions might also take the permanent URL of the blog entry and stuff that in the discussion body to link back to the blog.
Taking it REEEEEEEEEEELY far, some freak of nature might use an API to build a completely different UI, say a native application that interacts with Vanilla like webservice. Freaky.
From the progress I have seen so far, vanilla isn't too far away from becoming it's own self-sustained API. In the foreseeable future, I can see the Vanilla we're using right now as a forum as an extension itself totally seperate with other extensions working inline with it. It's fair to say I've been looking over the source briefly and thinking this exact same thing, subterrane.
If there's a way to strip the forum meat off of the framework backbone it might be totally possible to do what you're asking for.