You might argue that the homepage of the Forums is the most important page, but there are equally good arguments for individual thread views being the most important. Data suggests people land on these "deep links" in higher numbers than the homepage itself, even though the homepage is the most visited individual page.
Right away we struggle with exactly where we need to change to control the markup for this page. I apologize for this, it's my fault for not having a better grasp of how to do this. But it doesn't bode well for how easy Vanilla Forums makes templating either.
We do end up figuring out the core issue: the file in our theme was in the wrong place. Once we fix that, we get proper control.
We end up with a bit more work to do on the look of individual threads, but we have all we need to get it done. They won't look exactly like the wireframes, because we're going to intentionally make them look exactly like the comments area on blog posts. Consistency is going to make things easier for us and better for everyone.