This is a total hack on my part since I
know little about PHP myself, and it may take a little work to make work, but this is what I would try.
First, I'd make my own vB powered page. This is actually way easier then it sounds and I highly recommend you look this over:
How to create your own vBulletin-powered page! (uses vB templates)
Once I'd created the template in that article, I'd remove all the stuff I don't need
from the template and put in my login code.
Then, I'd use an iframe to display the new login page I've created in whatever place I wanted it in WordPress.
Now from this point, I'd have to look at how the redirect is handled. I would think you should be able to find what to do after a search here. But you could probably have your forum's navbar block show in the iframe. Theoretically, that last part should work just fine. Hopefully that's not too far out of the realm you where looking at.