[eluser]litzinger[/eluser]
I agree with Derek. I was trying to do the same thing with a site... use Wordpress for the blog and individual pages, Vanilla for the forum, and eventually a separate CI app, and I found that setting up the Wordpress template, and trying to manage the same template and navigation in a completely different environment (Vanilla) was not fun. They're all great apps individually, but I'm using EE Core right now with plans to get the Forum later, and it's much less of a headache. I had a headache even before contemplating how I was going to have a single user sign on area between the 3 apps. Now I'll eventually have EE manage the users, then tap into that user list with my app.
If I were you I'd try EE for the pages, blog, and forum, then your app is the only standalone thing on the site so there is less to maintain. I think you'll thank us later