Check out JFusion at jfusion-dot-org. (Sorry, I apparently can't post links yet. ) When I was investigating Joomla as an integrated vB option I found that it's all you need. Nothing better, from what I read. Good luck, let us know how it goes.
When I first found JFusion, iFrame was still the way to accomplish some of the integrations, but I believe the newer version(s) of JF give you a better integrated ('frameless integration') option. From a cursory search on the jfusion forum I see that's the case, but there may be SEO issues involved. I guess it depends on how closely one wants to integrate the two products. If a shared user registration/login system is the main concern, it should be fine. But as far as integration options go, I don't think there's anything out there better than JF.
I agree JFusion is amazing, I have used it to integrate phpbb3 with my joomla site and It looked asif the forum was a joomla made component.
That's great to hear. I really respect what they've done with the product. I know that seamless integration is pretty much just a holy grail, but from what I've seen they appear to have put a lot of effort into the project. Quite frankly, it was one component of the Joomla platform that spurred me on to investigate the other options. If a certain Joomla-only product or two pans out, I hope to use that as a stand-beside a vB-based site. With only the user logins being the major issue, it could work. I can't speak, though, as someone who wants to run a fully-featured Joomla site with vB as the user/forum/comments backend. I think that's where the situation gets complicated.
There are a few really good joomla made forum components..They dont have as many features as VB but they will one day..Kunena and Agora..both amazing and made for Joomla.